When I first got my Cab it had a JVC player wedged in, connected to 3 speakers by some hapless soul who'd obviously never come across BMWs concept of audio.... it took me ages to work out where the wires did and didn't go, but eventually after 3 weeks I had the main four speakers rewired and the two tweeters in the front - the tweeters are the only thing using original wires, resoldered and routed in the footwells.
I had to cut out alu spacer rings for the 13cms in the footwell as BMW creates the baffle (front and back) by the driver resting against the trim in front, rather than being bolted into the baffle as it usual (and far better). So I now get some low midrange from a stock looking setup.
I used Apline 13cm components for the front, I didn't think much of the quality and may add a crossover and replace the 13cm drivers with Pioneers later. The pioneers are more efficient - you get about 2-3 times the volume of the alpines for the same power. Efficiency is king in a car, measured by dB/watt usually at 1m distance. You've only got 24-26w rms max power, per speaker. The back had the original cloth surround whizzer cones - as they were cloth surround instead of modern rubbish foam they were still good, but I replaced them anyway with Pioneer TS-G1316 whizzers. These are light, efficient, cheap and sound superb with rubber surrounds so they'll last forever.
I realized at about this time that despite having 13cm drivers, the way BMW had designed the mountings meant no bass at all. Nada. So a sub in an E30 cab is not a luxury - it is essential. As everything else looked standard still I bought a small amp (Alpine MRP-T220, I had to climb into the boot to fit that!) and hooked it up to a 10" JBL sub - rubber surround with max 1000W power - i.e. indestructible and pretty well made. Routing this all to a new MP3 head (actually a £50 Aldi special currently, I'm working towards a DAB Blaupunkt!) took time as all the trim on the left-hand side comes out with the back seats, glovebox, new aerial that works etc.
The problem I had with regular bass bins is that all that lovely bass rattles around all 6 sides of the boot, of which I only get to listen to 1 side. i.e. I'll hear 1/12th of the power into the car (1/2 goes into the bass box). This is part of the reason that car bass setups are so powerful but still quieter (inside the car) than a much lower power hi-fi. Passers by will hear much of the escaping 5/6ths bass energy.
So after a bit of routing to give the sub's big rubber roll surround room to breath I screwed a lump of 17mm smartboard to the bulkhead, and the driver to the woodwork. The wood has two holes only routed through the natural holes in the bulkhead. They are only about 2" dia - tiny. You'd be amazed how much sound gets through that! So it's quite restricted but in essence I get 1/2 the sound my way, 1/2 is lost (like any speaker bar horns). So I'm 6 times the power ahead of a bass bin in the boot now - the Aplines 140w RMS now sounds like a boot bass bin of 840w inside the car. The speaker wire was then soldered on and the original trim cut out a bit and popped back in.
If I'd used a 12" jbl I'd have got 3dB extra volume for free (twice as loud) but 10" is a really good fit, 12" would be a struggle there.
Future mods are maybe jbl tweeters to replace the alpines, possible move of the battery from the front to the boot + a small cap at the head unit, better head unit, enlarging the holes for the sub, padding the void between the sub and the seat with sound deadening (except where the sub is), and possible opening of some small holes behind the back seat where the sub is. I'll also get a switch to cut the aerial raise signal when I'm not using the radio, as you can hear the wind whistling past when it's up...
As it is however it sounds remarkably good, haven't managed to clip the sub yet and you can feel the back seat whizzing back and forward with the bass - it moves that much air. All the bass notes seem even - there are surprisingly no big peaks or missing notes. Plus loads of boot space and completely standard look inside the car. It would probably benefit from an IB enclosure but I don't want to lose the boot space + I'd have to make it..
so what your telling me is that i didnt have to take all of the interior out of my car to run the heavy gauge cable from my battery to my boot and fit an inline fuse? Gutted... why didnt i find this forum before!
or as i did in my first set up i used the spare speaker cable as a supply 
didnt sound the best!!!!