When I play a CD in my car CD player, it develops an oily film on the underside of the CD, making the CD un-playable, until I clean it off. The garage looked for leaking coolant under the dashboard, but could find no leaks.
High evaporation rate from all the fast-food wrappers stuffed down between the seats…
Seriously, how about a defective CD player leaking electronic slime…
Check the coolant level. You could have a very small leak in the heater. Do you get a sweet smell in your car when you turn on the heat?