My first try would be to research a longer cable assembly. Contact the OE manufacturer. I wouldn't compromise an install just because the cable's too short. Put it this way, you're probably not the first guy (nor last) that requires a longer cable -- I'm sure there's someone out there that can help you obtain a longer cable (or adapter).
You might want to contact Howard Blitz (owner) of Car Acoustics in Campbell, CA. They've done a lot of very custom (and expensive) installs. This should not be unique to him --> 408-866-1250.
Just my 2-cents. Good Luck.
[This message has been edited by Brian16V (edited 04-24-2000).]