I'm happy with the stock stereo, and I don't have much money to spend on buying a mod kit and a new stereo, so I decided to be an electrical engineer and figure out a way to wire in an auxiliary input.
I couldn't find anything online that gave a detailed description of what to do, so I am going to be painfully detailed here, with lots of pictures. I hope it helps everyone who's still looking for a solution to this challenge.
My first resource was this website, which provided a link to a PDF outlining how to remove the stereo from the dashboard (see page 3). It's pretty straightforward, but I will show you exactly what to do here. [Update: see the link at the bottom of this post for another PDF showing how to remove the stereo]
Before we begin, I do recommend you read through this whole thing before you start, just so there are no surprises. It will make everything go smoother.
Part One: Removing the Stereo
Here's the untouched stereo
My trusted tools for the job: screwdriver, flashlight, thin knife, double-sided tape, skewers, and scotch tape. With these tools you can do just about anything.
Before we start, it's important that you disconnect your battery. So pop the hood and disconnect the wire from the negative terminal of your battery. I skipped this step because I'm an
experienced electrical engineer idiot but that doesn't mean you should ever cut corners and risk hurting or killing yourself by accidentally causing a short.
Okay, with that done, pop open the glove box and remove the two screws that hold in the little silver plastic strip. If your screwdriver can't fit in the space, you can make the glovebox drop lower by unhooking those squarish black plastic pieces on each side of the glovebox.
The first screw.
And the second one.
Now this part is a little nerve-wracking (at least it is for me). With even pressure you need to start pulling out that strip, starting on the glove box end. It's attached to the dashboard by numerous small plastic clips that need to pop out.
Keep going until it's pulled out completely, past the stereo, just up to the steering wheel.
Now you can see there are these little white plastic clips that clip onto the strip, and into the dashboard. Fun fact about these little clips: they love to pop off the strip sometimes, and fall inside the dashboard. This is without a doubt the most frustrating and emotionally battering part of this process, since you don't want your dashboard to be all wiggly. Again, if you take care to pull the strip off gently you should be okay, but do expect to lose at least one clip.
In the above picture you can see that I lost a clip, but luckily I was able to find it with the flashlight after I got the stereo out. That's what the skewers and tapes were for. I constructed a long skewer with some double-sided tape folded over one end (gum doesn't work as well, I've tried).
Back to it. Now that the strip is removed, you need to remove the panels on either side of the stereo. They are each attached with two more of those white clips, and a single screw on the bottom. Remove the screw and gently pry the panel off the dashboard.
I lost yet another clip, as you can see here.
Repeat with the other panel.
With the panels pulled forward, you have full access to your stereo.
First remove the two screws that attach the darker plastic to the panels (i.e. don't remove that screw in the middle).
Now remove the two screws on each side of the stereo
With that, the stereo should slide forward with ease. It can't come too far out, so get ready to remove the power and speaker connector shown below, as well as the antenna connector.
The power connector has a little tab at the back which you have to press down before it will come loose. Mine was pretty stiffly in there, so I needed to wiggle it a bit.
Next remove the antenna cable. It was also very snug, so I needed to use a bit of force to get it out.
And there it is. Now you can completely remove the stereo and wave it around wildly (not recommended).
Now your dashboard has character!
Here are a few pics of the power/speaker connector, for reference. If you are interested in what wires do what, this link seems to describe them. [Update: see the link at the bottom of this post for a PDF showing this connector (24P) and its wiring information]
Finally, I taped a little reminder up so I wouldn't forget where the radio goes.
Part Two: Taking the Stereo Apart
I wanted to be a cool electrical engineer and figure out a clever way to wire up an auxiliary input into the existing electronics, which naturally requires taking the stereo apart. Spoiler: Nothing came of this so these next few steps are for fun only, you don't need to take the stereo completely apart.
Just one or two screws held the top panel in place.
Then a couple more screws held the CD unit in place.
I had to remove a couple of screws on the outside of the stereo for the CD unit to come out.
The cable that connects the CD unit to the main board just pulls out.
Leaving us with... some electronics stuff!
Essentially the system is made up of a microcontroller (which handles the LCD screen, buttons, functions, etc.) a DSP (which handles the audio processing), and a pre-amplifier system to power the speakers. While I maybe could have gotten something working eventually, it would involve hijacking the DSP or the audio output from the DSP to the pre-amp. I figured either method risks breaking the stereo circuitry or potentially providing a crackly audio signal neither of which I was prepared to deal with. So I chickened out and bought a cheap inline FM modulator from FutureShop. You might be able to get better quality ones for a bit more money, but my intent was to do this as cheaply as possible.
How am does work: Basically this solution sits inline with your FM antenna. The unit is set to a specific radio station, and when you turn it on the unit attenuates all radio frequencies surrounding that pre-set station and modulates its own FM signal at that frequency. It's the same idea as those cheap FM transmitters you can buy, however this solution won't experience the same interference from outside stations.
It's not an ideal solution as FM audio bandwidth is limited, and there are always losses involved with encoding and immediately decoding and filtering an FM radio signal. But it honestly isn't that bad.
Taking it out of the packaging and pulling it all apart we can see that its connections are an antenna input and output, as well as a pair of wires for power. They feed off the same ~12VDC supply the radio gets from the battery.
Since the Sonata doesn't provide a spare power connection behind the dash, I had to wire up the power on my own. Finally, an opportunity to prove my electrical skills! The ground wire I just mechanically fastened to the enclosure with a nut and bolt, and the power line I soldered to the back of the protection diode beside the connector. This connected me to the constant power line which I thought was most appropriate. To do this, you might want to just strip back the power wire near the connector instead of taking the stereo apart.
In hindsight, I should have wired the +12V line to the striped Orange/Black switched power line. That way, the FM modulator box could only be turned on when the car is started or the key is in the accessories position. The way it is now, this unit can be turned on at any time, which means that if I forget to turn it off before I leave the car, it will drain the battery. I can live with that, so I'm not about to take everything apart again, but I wouldn't wire it up the same way if I ever had to do it again.
Part Three: Putting Back the Pieces
With accessible power wires, we're ready to put everything back together again. Just to be sure, I hooked up the transmitter to the stereo without putting everything in place, just to make sure it all worked.
It did work, which is usually a good sign that it works.
While there was a part of me that just wanted to leave it like this, I decided it would be best if I finished the job.
I ran the wires from the box through a hole behind the glove compartment. I wanted to leave the box accessible in case I needed to change the default radio station at some point.
The wires don't seem to get pinched at all if they're bundled and fed through this hole.
I stuck the control unit near the light inside the glove box. I really wanted to somehow feed this through to the dashboard near the radio, but it wasn't looking like an easy job.
With everything tidied up, I did one final test before putting all the screws in, and reattaching the strip to the dashboard.
All was good, and a headphone cable can slip out of the top of the glove compartment without getting pinched badly.
Everything looks as good as before, and it even still works! I'd call that a success.
I hope this guide was helpful and informative! While I know this isn't the ideal solution to the problem, I do think it's the cheapest most practical solution for this car. I've been using the AUX input with my iPod for a month or so now, and I have to say it's pretty good. I'm not blown away by the audio quality, but I'm definitely happy enough with it.
Update: Thanks Mike for supplying these PDFs outlining how to remove the stereo and the connector wiring!