Car Kit

car kit
My car has a cassette deck which when hooked up to a discman car kit makes only one speaker work. Why?

The car is an '89 Corolla.
The cassette unit is older.
When I play cassettes, only the left speaker works and the sound is distorted.
When I play the radio, both speakers work and the clarity is great.
When I play a cd through the audio sonic car kit and my sony discman, only the left speaker works.
I have a friend who has the exact same problem.
What can I do to fix it?
Any thoughts would be greatly appreciated!

Take it from a pro ( I'm a technician ).
First of all, cut the power off the unit or pull it out if it's a drawer style.
There's a small switch that switches the audio wires between the radio and the tape to the amplifier part of the unit when you insert a tape. It can be either broken, or just dirty ( the contacts ). You can open the unit, and try to find the switch ( there is two of it, the other one is for the power wires for the tape motor, it's all black usually ), then spray it with a little of contact spray ( Sonax or WD-40 are ok ), then insert and remove a tape several times to actuate it ( don't worry about doing this to the wrong switch, it only improves it's job). Then wipe off any excess oil with a cotton bud or a cloth. wait 7 to 10 min. then try it.