If I'm not mistaken, there were only a few HU's ever made that this was possible so chances are you don't have one and I wouldn't advise it. What kind of front door speakers are you using and what kind of tweeters? Do the tweeters come with crossovers? I would suggest buying a crossover for the front mids/highs and buying a small amp to power them unless they are very effecient and could be run off the HU. If you amp the fronts you can run the rear door speakers off the HU.
