I imagine audio electronics is not MMC's niche skill. Nothing mentioned so far suggests other than two channel amplifier and haven't seen any bluetooth hookups that stream other than two channel audio anyway. If so absolutely no chance of external fader.
Heinz's impedance comments are relevant if front and rear speakers are just paralleled up to amplifier each side. Not just for total impedance load but, as already mentioned, for power transfer to each if front and rear speakers have impedance mis-match.
Simplest way to gain the flexibility CX owners appear to seek may be to junk the factory amp and fit Bluetooth/DAB car radio allowing dedicated connections to all four speakers.
