[Wongway]

I've also been trying to figure out how best to go with upgrading my base system.

I'm not sure of one thing though, if I go with a MusicarNW system, do I still need to get Technic's harness? Or will all their plug and play stuff include a similar harness?

[ctuna]

The MusicarNW kits often includes a Technic Harness
depending on the system you get . It will include some kind
of Harness regardless of what system you get. I think for
the base system the Musicar Harness does not require you take
out the radio the rewiring is done at the Underseats via there harness . Not really sure of the details . My Hi Fi kit from MusicarNW came with a Technic Harness. They really mean plug and play and they give you everything you need to install it but the tools and the know how. You have to buy your own sound deadening if you want that though.
I have commented before it would be good if they would put a small
instruction sheet and parts list in with there package, but it you
know the basics about audio and car audio you will figure it out pretty easily . Removing Panels and attaching things is the hardest part.

[E30 Racer]

It's for an E82, but you may find this helpful:

http://www.1addicts.com/forums/showthread.php?t=654680