His stock deck is almost completely non functional. The CD player does not work and the volume / power control arm is broken internally. The only way to try to adjust the volume is to use pliers on pull on the volume arm to turn it. It doesn't really work well as turning it clockwise will increase and decrease the volume. We got it set to a nominal level and use the 3 band EQ to adjust the volume up and down at this point
Until I take the console apart, I wont really know if he has a sub / amp in it or not. That was why I asked about the stock amp being proprietary to the stock deck.
If the stock amp is getting its input from the rear speaker wires, then that would not work well when I replace the deck that has (what I believe) will be an amplified output to the speakers (unless it is designed that way). You could probably add some info on that part with the experience you have with the system.
As I am typing this, I am wondering about what you said that the stock amp is getting its inputs from the rear speakers in the stock config. Using the rear speaker outputs from the new deck to the existing amp ( if it is there and if it is possible) may have an effect on the impedance on those channels in combination with the new sound bar speakers I plan to install. Amps typically use low level inputs. If this amp can use amplified inputs to drive, I dont expect it to affect the impedance of the rear speakers. But if it does and depending what type of impedance in would introduce could cause an issue with the new deck outputs. Paralleled resistance with the new speakers would change the 2 rear channel impedance. If the 4 ohm impedance is changed to a higher value, the new deck would not drive as much power to the sound bar speakers. If it is lower value, the 2 rear channels of the new deck would be driving harder and possibly cause heat / failure problems. I cant say for sure that this would happen. Most of the new decks only specify the output wattage and do not specify the expected impedance levels. In the past they have been 4/8 ohm.
Sorry for the long thought process. I'm just trying to think about all the variables that will / may come into play when we upgrade...