If you're going to go through all the work of re-wiring, put a new amp in. The new headunit doesn't have enough power to drive speakers to realistic listening levels in a convertible.
I pretty much copied this system:
Porsche Boxster JL Audio Stealthbox Style Subwoofer Upgrade - 6speedonline.com Forums
but could have gotten away with cheaper speakers. Sounds great and I can hear it at 65 mph with the top down...