Check Fuses on both the amp and the wiring kit. If they are blown replace the fuse and if a problem occurs still then most likely the amp is dead. Switching pos and neg is the worst thing to do. Chances are then amp burnt up and also if you started up the system and heard a loud pop then thatd be your sub blowing

That shouldn't blow the sub, it's just a different polarity, did you use it while it was hooked up backwards and did you use it when it was hooked up right?

Youd be amazed with the things that happen. Ive seen alot of bad come from little things and Ive been installing car audio since 2007. Switching the neg and pos usual just burns up the fuse thus needing replacement but theres always a slight chance the it can surge the amp causing a pop sound from the woofer which can also destroy the sub in the process

You are definitely correct, there is a good chance he could have blown the sub with the negative pops... Depends how loud he tried it right? Could just be a fuse or the amp too! You have obvious experience so I'll leave this discussion to you lmao

AchievedMore wrote So i have had my two JBL 15 inch subs in my car for a while and today my friend thought it would be funny to switch the negative and positive wire going from the amp to the sub so like it was backwards and i think it messed up the sub is it possible that the subs are blown??

it shouldnt have. This is what you call "out of phase" wiring. I accidentally ran one of my subs like that for a week before i fixed it.