Absinthe

Not sure the year of your car but AFAIK the head unit sends two pair of balanced audio information back to the Bose amp. Left Front, Right Front, Left Rear and Right Rear. The signals for the subwoofer are taken from the Bose amp and supplied directly to the dedicated Bose Subwoofer Amp. Sorry, can't comment on any aftermarket head unit replacements

DaNello

Yes, what I see in all Infiniti with Bose documentation is that the AV Control Unit has four output channels, Left and Right Front and Rear. The Front Left and Right are sent to Bose and the Rear Left and Right are terminated. I assume the Bose Amp takes the simple L/R stereo input and does its DSP, crossover, fade, etc. with that, within the amp, and with other user inputs sent by AV Control to the amp, for the amp to make those actual adjustments. I cannot tell if the AV Control outputs are powered/speaker level or line level. It would make sense if they are speaker level, as non-Bose diagrams show the AV's outs going straight to the speakers. I assume if the Tesla unit has to replicate the AV Control Unit's output at speaker level instead of line level, it would be more volatile or difficult to match, to get a good input to the Bose Amp. But it's beyond me at that point.

Longshot

Yes, the harness phoenix sent with the unit was very much plug and play and sent signal straight to the bose amp like the OEM. there was definitely an audio quality drop, but there are some settings that could be changed to bring it back to life a bit, the default audio fader setting in the hidden settings menu will make you think your head unit isn't working. I did add an aftermarket sub though to compensate - there is no dedicated woofer line though from the head unit