I first mounted mine under the back seat, on the driver's side. Then I completely customized my system. I have a subwoofer in the previously mentioned location, and I have two amps on the hump under the rear seat. I now have the changer (Kenwood 10-disc) under the front passenger seat. It wouldn't fit under the driver's seat due to the wires and motor of the power seat.
I fabbed a bracket. I just used wood. To mount level (side-to-side) I mounted the wood under the seat frame on the middle-side of the seat, and on top of the seat frame on the outer portion. I am not completely finished. I am later going to make the bracket out of metal, and use rubber grommets between this piece and the brackets that attach to it and the sides of the changer itself.
I hope this helps.