The reply before this one was the best and the others have thier place as well. Probably less than 1% of audio competition cars have rear speakers at all except subs and they are up front as well is some of the best(in the rear of some of the other best)
Normally out of phase for A pillar tweets is the correct solution but not always as pointed out already here
Next lay out some towels on your dash and take a good listen, then remove them and try again. Use your best sounding CD's for this and play it moderately loud to medium loud for best results but not really loud and do it while parked with the engine off.
Most vehicles benfit from those ugly dash mats, too bad, I use them myself though.