It will be a bit more involvedim terms of wiring, but you should be able to wire in a crossover to a set of speakers (like the doors) from behind the dash. Then run your exterior speakers from that. You can use the crossover to toggle them on and off or mix the levels however you like. Depending on what sound/volume you want, you may want an amplifier anyway, many of which have a built in crossover. If you hunt, you should be able to find one that is easily controllable. An equalizer may do the trick for you as well. Same kind of idea. I'm not really into the stereo world anymore, haven't been for a long time, so I'm not too familiar with the products that are out there. There's no reason you shouldn't be able to do this though.
Good luck!