blown stereo and amp
 

Hey im a beginner in the stereo world, but ive hooked up an amp and stereo quite a few times with no problem. I drive a 1972 ford f250 with a custom speaker box that has a 12in sub, that i was trying to hook up to my 500w amp. I used to have it hooked up, and it worked fine, then my amp blew. so i tried hooking up a the 500w i have, much nicer, but it didn't work. tried it in my buddys car, worked fine. tried it with my wiring on different subs, didnt work. So i was messing with it again today, and i couldn't figure anything out, checked all my wiring, had power to the amp, nothing. so i tried connecting the sub to both the amp and the stereo, when i touched the stereo wires to the sub, i heard a quite but high pitched noise, then my stereo and amp shut off. i thought i blew a fuse, but ive checked every fuse i can find and they are all fine.
I tracked the stereos power wire back and it lead to a black box that had two ground wires, and a wire going to the solenoid. I thought maybe that was bad, and if i ran straight wire from the battery to the stereo it might turn on, but i had no luck.
so my question is, did my stupidity of trying to hook the sub to both the stereo and amp blow both?

So you tried to connect the outputs of your deck and the outputs of your amp to the same terminals on a speaker? I expect that would smoke something.

yes thats what i did. But no fuses were blown, i have one going from the amp power to the battery, and one that is on the stereo, nothing was blown. i searched for any other fuses i could find, there was nothing.

The amp may still be good try it separately with another deck, as for your current deck that should be fried with all power line fuses still good as you have discovered, because you have sent a higher voltage back through it's speaker output, assuming I am reading you right.