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Just to answer the sub question....
The factory amp is a 6 channel amp.
2 front channels
2 rear channels
2 sub woofer channels
Sub has dual voice coils with one channel of the amp per voice coil.
How long ago did you install the Kenwood HU???
I would go back and RECHECK all the connections behind the HU.
I see... so issues were there prior to HU swap... I would definitely suspect the amp then.
If it is amp, perfect time to go aftermarket They make amps small enough to fit in that same space, especially if you dont need the bracket.
You mentioned the rear camera not working some times. That has nothing to do with the amp.