S3 2002 bose system retro fit

Might be a better post in the 8L S3 forum, but maybe someone does know on here as there a few 8L S3 > 8P converts ?

That's a major job.
None of the wiring is the same so all of it has to be replaced. So that's the loom from the HU to the boot, including the front/rear pre-outs power for the amp and the speaker loom from the boot to the front & rear speakers. If you have the looms then it's still a painful task. If you don't have the looms then forget it. The upgrade is pretty pointless anyway as the Bose system is only slightly better than the standard system (have you looked at the Bose replacement rear speakers? They are presumably taken from an old WWII valve radio).
You'd be much better going aftermarket, or at least adding just a sub (or stealth sub) and upgrading the front components. Much less work and it'll sounf a million times better.
Probably not the answer you were looking for, but at least you know before you start that it's a pretty hopeless exercise.

I wouldn't bother with Bose having had Bose and non-Bose. Get yourself one of Andy's stealth subs. It sounded a lot better than Bose.

i agree. although the head unit looks good if you wire a decent amp and sub in the system actually transforms really well.

Apologies for the blatant post hijack, but I wondered if anyone who originally had a BOSE system in their A3/S3 and has subsequently upgraded the front speakers as AndyMac suggests still had them spare?

I've blown the driver's side woofer and would like to replace it with another BOSE item if anyone has one spare. Thanks.

The HU is identical whether Bose or non Bose, it's just got a flatter EQ (set on the back by grounding the Bose/GALA pin), thus the absolute lack of any real variance when you up the treble or bass settings.

On the sub front, yes the Bose sub is better, but not punchy in any way shape or form. Woolly is about as good as it gets and by replacing the stock non Bose sub you lose the rear speakers as they run off the same amp that's inside the sub.
I tried exactly the same thing before designing my stealth sub. The Bose sub is more powerful at 100w, but it's the enclosure that's the problem, it uses the same tupperware box as the non Bose unit.
So you'd need to customise the sub loom to get the feed and wiring into the Bose sub (as the plugs are completely different), then extract the amp from the non Bose sub and link it up as well to give you the feed for the rear speakers (or buy a small 2x25w aftermarket amp and use that). You'll then be straining the power cable on the non Bose loom as it's only rated at 5 amps, so you'd need to run some additional power. For all that work you'll have a marginal improvement in more woolly bass.