I am doing this right now since my wire harness got moshed. both my front and rear speakers are fed from my rear output.

The sound has less punch, but doesn't sound too much quieter. I have not adjusted the gain. I plan to fix this this coming weekend.

Edit: I've been FAQ'd with this one before. See diagram #4 on the How do I run a subwoofer if the player doesn't have a subwoofer output? topic.