In the event anyone wants to install a subwoofer, I have some information that may be useful to you. I don't see any similar information here in the forum so I decided to make this post in case anyone is interested in doing something similar. I decided to install a Kenwood KSC-WD250 that Lisa and I had, leftover from her Civic. I looked online for electrical diagrams and came up emtpy. I did find some information on the basic four speaker stereo, but that was not useful to me as we have the Boston Acoustics setup in our Compass.
I took some interior panels off in the rear of the vehicle to see what I was up against. I found that the outward grill is about 4-5 inches away from the actual subwoofer that Boston uses. Some good pictures of this can be found in this very forum. With the panel removed, I took the floor panel out as well for full access to all the wiring. There is an amplifer that powers the Boston sub mounted vertically right beside the sub itself. I removed the incoming wires (pre-amp) via a plug, and pulled out my multimeter. Here's what I found:
Seven (7) total wires pre-amp.
-Two twisted green wires, one with a white (or gray) strip and one with a lighter green stripe. These are incoming speaker (signal) wires.
-Two twisted white (or gray) wires, one with a dark green stripe, and one with a light green stripe. (this light green matches the light green found on the one green wire above) These are also incoming speaker (signal) wires.
-One black wire with a blue stripe. This wire is ground.
-One red wire with a green stripe. This is your constant 12V power source.
-One white (or gray) wire with a red stripe. This is your 12V switched power source.
I installed the sub by hooking the four speaker wires up (using the light green striped wires as negative since they matched) to my amplifer. This Kenwood combo unit accepts speaker-level signal as well as RCA.
I hooked my amplifer's power wires to the red with green stripe wire.
I hooked my "sense" wire, the one that turns the amplifier on and off with the radio or ignition, to the gray with red stripe wire.
I then grounded my amplifer to the main ground point in the rear of the vehicle. There are about six or seven wires already grounded here, including the black with blue stripe wire mentioned above.
Worked well! The sub/amplifer combo has a wired remote which happens to fit perfectly into the pocket in front of the cup holders while still retaining some usable space in this pocket.
Overall it sounds good. It fills out the deep bass that the Boston system lacks, which the 8" Boston sub cannot provide. I taped off the source plug that was plugged into the amp, and also taped off the receiving plug on the amp itself. So for now, the 8" sub is no longer being used. I now have a 100 watt (RMS) 10" Kenwood class D amp pushing the 10" sub in the box.
Pictures can be taken upon request if the need presents itself.
On a side note, the box can be removed in less than 60 seconds if additional storage room is needed in the back of the Compass. I can easily still access the spare as well.
NOTE: This class D amplifer that Kenwood combo unit uses can easily be powered this way. Obviously I would recommend running your own power wire(s) if you are running a stand-alone amplifer that would require more current. The way I have this setup, the amp runs very cool after 30 minutes of loud music. I cannot feel any heat at all, maybe just a touch of warmth at max.
Hope this is useful to someone at some point.