Has anybody tried tapping into the head unit's signal yet to see?
Since nobody replied, I'm guessing that I'm venturing into unknown territory...
I was out of work early enough today that I could take advantage of the decent weather, so I broke out my multimeter and some old 4Ohm speakers I had laying around to check it myself. Here's what I found:
There are 3 molex-type plugs going into the Bose amp. They seem to go in order from left->right by wire gauge - - after a good bit of trial & error, I've mapped out the important wires.
1. Left plug - very small wires. (About Cat-3 size) Carries signal from head unit.
Yellow/Yellow stripe - Left Front
Purple/Purple stripe - Right Front
Grey/Grey Orange stripe - Left Rear
Green/Green stripe - Left Rear
2. Middle Plug - roughly 16 gauge wire, with a few smaller ones interspersed. Mainly speaker outputs -
Red with Yellow Stripe - +12v Amp Turn-on signal (from head unit)
Black - Ground to head unit
Yellow/Yellow stripe - Front Left
Purple/Purple stripe - Front Right
There were quite a few other wires on this plug, but it seemed that the rear speaker output used some sort of funky common ground? Since I'll be replacing the speaker wire anyway, I really didn't care... but I could probably map them better once I get the old speakers out.
3. Right Plug - Largest wires. Will mostly not be using these, as I'll run my own power from the battery & stiffening capacitor. There's about 7 wires on here, presumably having something to do with the subwoofer power.... but the ones I confirmed are:
Red/White Stripe - +12V always on
Black - Ground to car body
It took some creative wire-tapping, but once I got this all mapped, I was able to hook up some old 4Ohm speakers directly to the head unit. As best I could tell, the signal coming from the head unit is clean. I'm not positive, but I'd suspect that the head unit would be the same as if I didn't have the Bose system - - I don't have a non-Bose 500 head unit with which to compare, but I suspect it is designed for rather low-impedance speakers. I could get my 4Ohms to play, and they sounded pretty flat & clear, but they were MUCH quieter than I'd have expected. These were from an old factory system that I'd removed, so it's not like they were high-power speakers... but I had to crank them to 3/4 volume to get a decent level. It'll be interesting to see if I can "trick" the head unit into not thinking it's connected to a Bose system, and get that hidden EQ to come up!
So I'm looking at the AudioControl LC6i as the LOC to convert that signal into something my amp can use. Still have to take a few measurements and figure out what can go where... but I'm now pretty confident that I can keep the head unit (along with all the Blue&Me features) and replace the rest to end up with a much better sounding system.