I am planning to add a single sub to the stock Bose system (without navigation) in my 06 Max SL. How difficult is that and where should I tap into the oem wiring for the amp?

I'll probably want to upgrade the Bose amp for the doors later on so any advice on that would also be appreciated. Thanks.

 

You can get all of the signal and remote turn on from the amp in the trunk. All you need to run to the rear is the power wire.
You will need (and want to) upgrade the speakers in the doors if you change the amp that powers them.
Just run a 4 awg power wire, and you can use a fused distribution block when you add another amp.