Dunno if this will help but here goes. (does this belong in ICE now?)

Stock its like this: 6disc indash player full DIN HU (damn hu)
to "iso T header", to ISO header, to speakers. The T is a tap
of the rears to an underseat slimline self-amped sub. Not
a very good one, but audible none the less.

Now I've only changed this to an aftermarket 5 yr old cd deck
with an aux 1.2 din with my mk2 above it. No real changes.

What I'd like to do is mk2 to amp, amp to 'T' to speakers.
The T is identical to the factory iso header with the addition
of the tap for the sub. (then use the empty 1/2 din for
more gauges)

However unlike many cars, the WRX has 4 5"+ front and
rear in the doors, no dash, no rear-deck. Plus a pair of
mini-tweeters crossed over off the fronts later in the wiring.
So the sound is fairly "square" and does its own crossing over. There doesn't seem to be the high/front low/rear that
most vehicles seem to have. This doesn't deafen rear-seat
riders, but seems to lack some punch and range.

I was planning on using a 4 channel amp mounted in the trunk
under the rear-deck as its blank save for seatbelt tensioners.
use the amps built ins for a tiny bit higher front and lower
rear with the factory sub still attached. Then I could
perhaps boost the bass, but that leaves me wondering how
to protect the rear speaker without adding another crossover.

I don't tend to muck about with the fader either so I may play
at using the rear line as a seperate sub feed. Maybe a
box in the trunk is in order. The factory sub is rather lame.

But how to wire out 4 speakers off one line in? I need to
go do some serious amp research I guess. Didn't think
if you used 2 line ins you could split the outs like that.

Perhaps one of these nice 4channel+1s that are out,
drop the T-tap and run the sub directly off the amp. that
would fix a lot of problems.

Ed