Congrats! FWIW I think the V6 is underrepresented in the enthusiast forums but in real life, I'm sure you'll notice lots of them driving around, as it's unbeatable car for the money ;)
e30, wish I could give you all the details but I had a stereo shop put it in for me. I'm pretty sure the sub power hooks up directly to the battery. Here's the Mtx manual for what it's worth: https://www.mtx.com/v/SupportFiles/165/FMUST05BK10.pdf