Another spot would be on the side panels of the rear seat IF you have the 2dr GP. I have seen a few folks cut out the area and fashioned their own bracket to fit a 6.5" speaker there. Looks pretty sharp when done properly.
Frankly I would NOT recommend putting them "under" the rear seat. It will muffle and distort the sound rendering them uselessI replaced my Bose 5.5" front door speakers with a pair Memphis Audio M-Class 6.5" speakers. I just dremeled out the bracket to fit the speakers and walla nice upgrade to the front speakers!