80 Chev Blazer eats Head Units

sounds like a headline... anyways  i have a cheesy cyclone brand head unit that ive had in my truck for a long time, it skips everytime you hit any bump, but  it played fine, and then i got a pioneer head unit from a friend, and he thought it worked, so i installed that, and it will turn on, and plays a cd, but no sound. i thought i may have had the + and - speaker wires mixed up, so i swapped em,  still no. And it eats cds and wont spit em out. So i put the reliable old cyclone back in, and now it wont even turn on. checked all fuses and connections. they are all good. Anything else i can do to fix the pioneer, or further test the cyclone?? Thanks