Cause: Your car is 30 years old
Solution:
Add an aux cable to your stock head unit. The tape deck will continue to work if the aux cable is not receiving any input.
To start off with, here's a video of it in action: https://www.youtube.com/watch?v=wVpk8iPJjyc
The audio is a little grainy because my output connection wasn't great. In the car it sounds like perfect digital audio.
Here are the steps:
1. Remove the top cover and expose the circuit board
Remove the top black cover to reveal the inside of the radio. There are two small screws on the side and two on the back. You will need to basically pry the back part of the panel up to clear some pins so that it slides back and out. Don't worry, its steel, you won't damage it.
If the front of the radio is up, we will be working in the top left corner, basically right behind the LCD screen. There may be an orangish/red piece of cardboard covering a circuit board, remove that.
Here is a summary of the board you're looking for and the pins we'll be working with. Look for the orange connector that has 4 pins. This is the input from the tape deck reader.
2. Remove the solder from all four pins
I removed the solder from mine, if you're confident you can just reflow the solder and stick the wires in.
You can remove solder using solder wick: https://youtu.be/VxMV6wGS3NY?t=3m33s
The rest of the video has good instructions on how to solder and unsolder components.
3. Solder on the aux cable
I recommend using a high quality aux cable that has a good shield and individual wires for left/right/ground.
I used this one which I was really happy with: https://www.amazon.com/gp/product/B01CUWZ1NQ
Cut off one end of the cable and strip the wires and shielding down to show bare leads. You don't need to do too much, just enough to solder onto the board.
The four pins coming out of the orange connector are:
* [Front of the radio]
* Left
* Right
* None - This one will be unused
* Ground
Refer to the image above again if you need a visual guide.
4. Isolate the wiring from the circuit board and secure the aux cable for strain relief
I put tape under the aux cable to prevent the remaining exposed cable from touching any other points on the board.
Then I ran the cable towards the back and taped that in place to act as strain relief so that the solder joints are not taking any stress
5. Reassemble the radio and run the aux cable out the back
Put the cover back on, don't forget the orange cardboard if you care about it (its not necessary, I believe its a dust cover)
Run the Aux cable out back like so (CM5905's need to run out the back. CM5907's can go out the side or the back due to an additional gap):
6. Done!
You now have an aux cable. You can either put in an empty/stripped out cassette tape, or there is a small white tab inside that you can push with a pen to make it go into tape mode (you see me push that in the video above).
The aux cable can be run out the side of the center console and into the glovebox to a bluetooth reciever or down by the shifter and out the bottom under the window buttons, any way you want to do it.
You may hear buzzing/static on the speakers when you install it. This is because of a ground loop due to the audio wiring harness having inconsistent grounding. Get a ground loop isolator and the buzzing should go away: https://www.amazon.com/gp/product/B019393MV2
More pictures for your edification:
