I currently have the rear seat out of my 2011 TL and believe I found the ground connection for the factory amp (lower back of passenger's side). There are three or four other wires grounded to this point. I haven't buzzed it out yet with my meter but will do so to be sure. I'm thinking this is a good place to ground my aftermarket amp to avoid ground loops. Or at least near by.

 

sand down some paint and create a new ground point. Always better than worrying about using a shared ground and introducing noise.

 

I've never heard of a common ground causing noise. I read that connecting the amp case to the vehicle chassis can cause noise, so I'll be isolating it using rubber grommets.

 

Turns out it wasn't the amplifier ground. The amplifier is grounded to the rear of the trunk underneath the trim panel. I'm curious how much of a difference it would make to run a ground wire from this point up to the grounding point of my LOC.