used cassette car stereo?

Bought a pioneer car stereo cassette player for 30 bucks at a flea market. had the seller test it in front of me and it worked fine. got home and installed it in my car and it worked fine. Took a spin around the corner and the sound slowly went down the off. Power was still there but no sound. cut the car off and the sound came back. for some reason the unit while car is moving will not play sound. If car is off sound returns to the unit and plays fine and all features work. What the devil is going on?