well, there are tons of things it could be, and it kinda depends on exactly what equiptment you have... all OEM? all aftermarket? a little bit of both?
if OEM, your radio is probably bad, could also be the amp, but the rear speaker thing makes me more likely to suspect the source unit.
if all aftermarket, again first look at the source, unless the subs and rears are handled buy the same amp.
fuses, fuses, fuses...
is it a little bit OEM and a little bit aftermarket? then it could be a wiring issue, maybe the LOC is shorting out (the rear speakers) and then not getting a signal for the subs.
start at the end of the chain and go backwards until the problem is found. do the speakers work if something else is feeding them > does the amp work if feeding it a different signal, does the amp work if feeding it a new signal from the sources end of the connectors...
also if you really have no clue how this kinda stuff works, then you might want to find a local shop that will trouble shoot it for $20-$40 and have it found out in no time.