Power comes through ground

Sounds like something is shorting out and it is in protection mode. Check that the wiring is correct and there are no bad spots on the power wires. Unhook the power before starting your evaluation and leave disconnected until you think you've found the problem then rehook the power source. This will give the system time to reset if it needs to.

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left positive and right negative-make sure the speaker that you intend to run in bridged mode is 4 ohms or you will kill your amp

try grounding the rca shield to see if it stops, if it does you have an rca ground problem in either your head unit or your amp

sounds like maybe a speaker short, try unhooking your speakers and then powering up your amp, if it then powers up with the speakers unhooked try hooking up each speaker one by one untill the amp goes back into protection and then you have your bad speaker.

Normally bridging an amp is using the left + and the right -
Here is a link to your amp specs

http://www.crutchfield.com/p_158XM2200/Sony-XM-2200GTX.html?tp=115&tab=features_and_specs

Usually the Left + and the right - is bridged. Here is a link to the specs on your amp:
http://www.crutchfield.com/p_158XM2200/Sony-XM-2200GTX.html?tp=115&tab=features_and_specs

I'll assume that you've checked the amplifier's on-board fuses.

With your multimeter set to DC volts, the black meter lead on the ground terminal of the amp (not on the point where the ground wire connected to the vehicle) and the head unit on (so the amp will have remote voltage applied), touch the red lead alternately to the B+ and remote terminals of the amp. If the voltage is below ~11 volts, you need to check the wiring feeding whichever line is too low.

Sony generally drives their power light from a source that does not rely on the amplifier's switching power supply. This means that it's not a good indicator as to the health of the amplifier.

 

As long as you have B+ and remote voltage, it will light up. Since you've tested the speaker and the signal source, I think the amplifier is likely defective.

Check your ground wire! Relocate it to another place in the car and try to get a heavier ground wire aswell.