Listening to music while you drive is very helpful, but unfortunately MP3 inputs for cars did not come out until the later 2000's. Using a cassette tape adapter usually works all right, but the quality is usually quite bad (and they are usually not built to last). An FM adapter is another option, but these can get pricey and you have to worry about a radio station using the frequency you are tuned to while traveling (I drive roughly 2.5 hours to school). Aftermarket radios are very expensive and they are a hassle to install. In my opinion, aftermarket radios just don't look quite right because they are meant to fit a wide range of vehicles. My solution uses the cassette deck of the original factory radio to play nice clean audio from your phone or MP3 player.
-Small Flathead Screwdriver
-Torx Set and Allen Wrench Sets
-Cresent Wrench (or correct size wrench for radio ground wire, possibly 10mm)
-Hot Glue Gun
-3.5mm Jack (radioshack has them)
Step 1: Remove the Radio
1) Disconnect the battery.
2) Turn ignition on and shift to drive (Make sure car is properly blocked!) and remove the shifter knob by removing the small allen screw directly under the shifter button.
3) Gently pull up on the trim that is under the shifter, it is just clipped on. You should have enough room to just set it aside without stressing the accessory and light wires (mine had wires for the heated seats that I added as well).
4) Remove the trim that covers the radio gently as well. You will not be able to pull it far from the dash and you will have to remove the connector on the traction control button and the connector on the climate control system.
5) Four phillips screws hold the radio in place, remove them and slide the radio outward.
6) Use the small flathead to remove the black and grey connectors from the radio.
7) Remove the antenna connector and use a small wrench to remove the ground strap.
8) Now the radio should be free!
Step 2: Disassemble the Radio
1) Remove the four screws that attach the front facing
2) Carefully disconnect the two wire connectors from the facing
3) Remove the screws that attach the tape deck
4) Gently lift the tape deck up (mind the wire as shown in the picture)
5) Remove the two screws and connectors that attach the tape deck PCB (printed circuit board)
Step 3: Make the Modifications to the Radio
Here is where the fun begins! So the tape deck basically works by reading the signals from the cassette tape. These signals are then fed through a preamplifier that is shown in the center of the picture. All I did was tapped into the output of the preamp; I found the data sheet for the preamp in order to solder the wires.
Note: Rather than soldering to the fine pitch pins of the preamplifier, I used a DMM to determine where the traces went and soldered to a more convenient component or test point.
Use your small wire (3 wires roughly a foot and a half long each) and solder one to each signal line (as shown with the white wires in the picture) and solder one to the signal ground (the black wire in the picture). I used shrinkwrap on the wires to hold them together nicely, but electrical tape will work just as well. Reassemble the radio, but do not connect the tape laser reader (small tape wire that leads to the cassette PCB). This will cause no sound to come from the tape in the cassette deck.
Step 4: Modify the Trim Panel
Measure twice and cut once... You will need to decide where you want to install the 3.5mm jack. Make sure that it will not interfere with any dash clips or rub on anything! I decided to place mine just to the left of the radio. Drill a hole about the size of the collar of the jack (you can unthread the little nut it comes with). Use hot glue or some sort of adhesive to attach the jack to the backside of the trim panel. Doesn't look half bad in my opinion! Note: it may not be a bad idea to hook up your radio and test it before you drill any holes.
Solder a ground wire and two signal wires with roughly six inches of wire to the audio jack. You will have to look up the pin out for a headphone jack, as the pad closest to the collar is ground and the other two are signals (left and right).
Step 5: Test It!
Solder connectors on each of the wiring harnesses. I just used simple male and femal headers that I trimmed to three pins. You will want to trim the wires as short as possible but still have enough room to install the trim. A little bit of trial and error was used here, and the reason for shorter wires is to reduce the amount of noise picked up by the small wires. Install your radio and connect the trim piece. Now you can connect the vehicle battery and see if it works! Insert any tape to trick the cassette deck into thinking there is a tape inserted and you should hear your MP3 input!
Step 6: Reassemble
Reassemble the radio and trim in the reverse order of the first step. The audio input looks nice and clean here!
Step 7: Conclusion
The music stopped every time the tape reaches the end... This is quite annoying, so I purchased a blank tape from meijer. By disassembling the tape and removing the reels, I now have a dummy tape that will spin forever. This works great for my Intrepid, but I tried to do the same with a 1998 Buick Lesabre and the radio will not accept the dummy tape (as it thinks that the tape is broken and ejects it).
I may try the same setup with a CD player if anyone is interested (I have two radios that fit my Intrepid, one tape/radio and one CD/radio). Also, I am looking at developing a bluetooth setup in the future once I can get a PCB built for it!