Well, you are making me question your speaker wiring. Not that the speakers are "unhoooked" but that, perhaps, something is wired such that the output stage(s) in the head units are taking damage.

I've seen some people actually ground one side of the speaker, thinking "Everything needs a good ground", but in these systems, the speakers 'float' above ground.

OR..... you wired too many speaking in parallel.

OR..... You wired left and right together. (Saw one time where a guy put an extra speaker in and wired it between left and right and called it a 'Center Channel Woofer'.)