Why wont my cassette player in my truck work all the time?

[eight_kinds_of_hell_454]

I use the cassette player in my 96 suburban for a tape adapter from my MP3 player. Sometimes it will work and sometimes it wont. It may work fine one time and then when i shut the truck off and get back in it later on it will either work or it will say "ERR" and then pop out the tape and go to the radio. Ive tried the special mode that the owners manual talks about for some kind of metal tape or something like that and it doesn't help. I've also tried the cleaning cassette and that doesn't help either. Any ideas whats wrong with it? The CD player and radio work fine all the time.