Sure, but generally people who give half a **** about audio talk about "sound reproduction"....that is reproducing sound. Generally from a concert or recording studio. In nearly every case sound originates from IN FRONT OF the observer. Sure you can mount speakers wherever your heart desires, but I'm not sure what the point is.
I've been into this hobby (and driving legally) since the late '80s. I remember some God awful factory speaker locations, and certainly remember when it was vogue to do the 6X9 boxes on the rear deck..... hell, people still do it. It's loud but sounds like dirt.
The only rides I've been in that sound good with mids and highs in the rear are with good 5.1 channel processing and ONLY with material that's recorded in 5.1. Otherwise it doesn't really reproduce anything resembling a concert or studio and I haven't even got into the extra phasing and timing issues that are bound to arise from multiple sources.
OP, unles you have 5.1 processing don't bother. Add more stuff up front if you need it louder, or use more power (or more efficient speakers) or work on better locations for your speakers or sound deaden their mounting locations. Or if you like to listen to your music with the lead singer in front of you and behind you at the same time, knock yourself out. You're the one that has to listen to it.q