When wiring a stereo jack socket should both negative [shield] wires be twisted together and soldered to the jack socket or will this create a ground loop? and therefore hum.
The kit I am using is a Cana kit [CK194 20W bridged]and both amps are of the same board. I have tried it with just one of the shield wires connected to the jack socket and it appears to work OK.
If I remember correctly the other wire is ground, so that should be soldered or at lest temporarily connected to the frame of the car, try it and if it works, then that's what it is. If not, just have it rigged the way it was.