The amps in the trunk are BTL amplifiers; neither speaker wire is supposed to be grounded, and it will kill it to do that. Both wires are at half battery voltage, and move in opposite directions to make up tp a ~8V max signal to the speaker. This allow a amp to put out twice as much power as it would with only 12V. You can get a max of about 36 watts at clipping from a 12Vbtl amp, in a 4 ohm speaker. Now that car speakers have moved to 2 ohms, it can be a max of 12^2/2=72W. This is at clipping, so probably more than 10% distortion; it would sound awful at that level.
Real amps have switching power supplies to make more supply voltage. Doubling the voltage increases power by 4x.
My amp has +-120Vrails, so 120^2/4=3600Wmax, per channel! lol. it uses a different audio stage, that wastes more voltage, to prevent Distortion; not running "rail to rail" makes audio amps "hifi"amps, and drops distortion below 1%, which most of us will never notice.
Grounding a speaker wire in a 'bridged' system runs one of the rails to ground. assuming it doesn't immediately kill the chip, it's running 36W of dc power thru your speaker. That pop you hear when you ground it is the speaker moving all the way one direction, just like hooking a battery to it.
Don't do that, it will kill it eventually. You have something wired wrong, I'm guessing it's a wire at the deck/amp connection.
Both speaker wires for a channel should go the the two terminals of the speaker. Decks can be "differental" out, same as described, or they can be single ended, which uses ground for one wire. Amp inputs usually want the single ended signals, that's why there are converter boxes to add an amp to a stock factory system. If you run a speaker level signal into an rca input, it will sound really bad. Hope this helps!
