If your car has Bose, be prepared to rewire the entire audio system and speakers 
Or just fix a 6 channel amp capable of driving 2 Ohm speakers. Fit the amp under the rear shelf where the bose amp is located as you'll have direct access to all speaker wires.
Carl,
I don't know on the Bose kit in these cars but generally Bose uses a lot of signal processing to achieve a good sound / protect the speakers - unless your amp can replicate this I suspect it will not sound good / destroy the speakers.
Steve
Correct, its a full active system, one Amp per Speaker (x 6), except the Tweeters which are just trimmed off the Front Mids via a crossover.
Bose do piss around with the EQ quiet a lot including Compression at different Volume levels, supposedly to match each individual Car Manufacturer acoustic interior.
They've been doing it for donkey years in the PA and Hi Fi world, claiming there's no point in producing a Flat Response Amp when there's no such thing as a Flat Response Speaker, so tweak the EQ of the Amp to match flaws in the speaker (generally a boost in lower and upper frequencies).
So yea, dont expect it to sound the same by introducing non Bose components.