You are going to need to locate the rear washer hose and see if it is leaking anywhere.
if the rear hose comes of a T from the front hose and both squirt when using them then it may be a pressure imbalance.
if there are 2 different pumps have someone run the rear and see if you hear a buzzing near the motor. you can tell what it will sound like by running the front pump.
it could be a stuck valve that is closed.
it can be a clogged hose, pump pickup, or squirter. If it's the squirter then you can stick a very small pin down it to dislodge whatever is blocking it. If it is a blocked pickup then you will need to remove the pickup from the tank and remove the block. a blocked line is more difficult but can be fixed by blowing compressed air backwards through the tube, make sure it's disconnected on the other end though so you don't damage the pump or valves.
you can go to a parts store or call a dealer for parts locations and where the hoses are routed. Usually they will be able to pull these things up on their systems. once you get the list of locations go check all the above. you can also search google for schematics and locations of parts.