A non-Bose head unit on the 3 (and other Mazdas in the same age like the CX-7 and 6) has a circuit designed within itself to roll off frequencies in the rear channels. This means that if you tap into the rear outputs (without correcting for this aggressive roll-off problem) you'll have sub-par bass output to say the least.
The way to fix this on the cheap is to use the front speaker outputs instead, which are slightly more flat. The better solution is to equalize the signal coming out of the front outputs to a flat or near-flat output. To do this, there are a few products out there that can work. EQ's will do it, and O.E. Integration products like the 3sixty.2 I'm using will also do it (and about a thousand other things!)
From cheap to expensive, you've got:
Standard LOC ~ $15-25
MTX ReEQ (an LOC that has a circuit designed to restore some of the lost bass) ~ $100
Audiocontrol or other brand of quality Line converter/EQ ~ $250+
advanced O.E. Integration device (JL Cleansweep, Alpine's new one, JBL's new one, RF 3sixty.2, etc.) ~ $300-600+
Hope that helps a bit.