Yeah, that's the thing. If it'll fit in the ashtray, put it there or in the glove compartment.

Or, you could run your two sets of pre's out to the amp for front & rear speakers, buy a Bazooka sub tube and tap off the rear speaker lines. http://www.bazooka.com/productAutoAmplifiedBTs.asp The amplified Bazookas come with a built-in x-over, hence why you can tap off your rear speaker line from the radio. This won't hurt the head unit, your rear speakers, or the Bazooka tube. I actually ran this setup for years and loved it. My current Alpine has front, rear, and sub outs. I'll pro'lly run the front and rear speakers off the Alpine and take the sub pre out to an amp in the trunk and run a sub off that. The only thing that limits us sometimes is the space, as you've seen with the EQ.