Hi lpj,
The sub is in the lower tailgate. I suspect the easiest way to install one would be to tap into the speaker outs from the amplifier (in drivers side luggage space panel) and then run that into an additional amp and then run new speaker cable to the lower tailgate and new sub.
Here's a couple of photos of what the lower tailgate looks like in a D3:
https://www.aulro.com/afvb/images/im...014/06/140.jpg
https://www.aulro.com/afvb/images/im...014/06/141.jpg
As you can see - the issue is mounting depth. I installed a Pioneer shallow mount sub in my previous D3. I am about to install a Dynaudio 10" in my D4.

