rreign

Well, to start this out, I am no expert so to speak. I have been installing car audio and doing custom fiberglass for about 5 years though. You can basically do what you want as long as you know how to offset your co-axial speakers. i.e. delay, mixing, fading etc. I have yet to tear into a 370 but I have done some similar sized cars and smaller. Your best bet would be to ditch the spare, build a custom enclosure to the spare tire well and mount the amp separately from the sub (as to not mess with the interior components from high decibels. Also because there isn't much room, mount the additional amp across from the other.

When you install the co-axials do all of your adjustments from the head unit to the way you want to hear your system. You would probably want more of a delay to the rear speakers and make sure you can set the mixing on your head unit. Probably like a 60% (front) 40%(rear) set up. Then turn up your head unit to about 75% of it's volume, then adjust the gain on your amp until the sound coming from the speakers starts to distort. When you can hear a little bit of distortion, back the gain down a little bit til it's clear again. This is your max volume and gain that your system will be able to obtain. Do the same with the sub, and you're done.

You can use basically any set-up you want. These are just rules of thumb to adjusting the system. Don't worry about glass or anything shattering from too much of anything. It's going to take way more than that 13 to do any harm to the car.

I hope this helped. If you need any more info or have any more questions just let me know.