Installing a cd player in 99 Camry

I am having a terrible problem trying to get a cd player put into my vehicle. I have always installed my own cd players in my cars with no trouble. Then I got this car, and got fooled. Anyway, before I took out the radio/cassette player, it worked fine, radio played, I could play tapes anything. Then I took that out and tried hooking up my own cd player into it. I bought the harness that was needed from crutchfield, and everything. I have wired everything together the way it needs to be but the cd player will not play a cd. It has power becasue it will take the cd in and out. So anyway, I gave up and took it out and tried putting my cassette player back in, now that won't work. I can't even listen to the radio or anything. Does anyone know what is wrong? I've had all kinds of people I know look at it, and no one can figure out what's wrong. Anyone have any ideas? If so, let me know, it'll be greatly appreciated!