Thanks for the interest, let me try and answer some of these questions.
1. Yes using Technic harness
2. No not using oem amp, the rears are being powered (albeit very lowly) by the hifi head unit. They do indeed provide fill this way, I learned it here!
3. Using Alpine 4 channel MRP amp. Made a big difference to the fronts/tweets and the underseats. Although I think would like more musical underseats.
4. Car is 2008 328
5. VP - I can get the ms-8 units used for ~$400. I'll shoot you a note on yours. Thanks.