Thank you americanaudiophile.
Since I installed an aftermarket head unit (w/ its own amp & used a Beatsonic interface), instead of buying another ML Factory Amp, I'm thinking of wiring the RL speaker directly to the new head unit (but only the rear left speaker because the instructions for the Beatsonic interface state to not connect the rear speakers to the new head unit or the amp/navigation will stop working.
I could control RL speaker with the new head unit and the RR speaker with the OEM controls (built into the factory navigation).
Does that make sense? I've replaced OEM head units with new after market head units before but never this complex with factory nav.
Thank you for any input/feedback you could offer.