A while ago, my rear wiper frequently wouldn't work when I hit the switch. Every time that happened, if I kept the switch on and went back and moved the wiper arm a little, it would start working then. My theory then was that since the rear wiper gets used so little (compared with the front wipers), the surface of commutator on the motor (the part that the brushes wipe against to flow current) was getting a little corroded without use and was making contact with the brushes a little hit or miss (I have seen this happen on power tool motors). I figured they just needed to be cleaned a bit which could effectively be done by just running the motor for a while and a bit more frequently. I ran the wiper for a long time and as frequently as I could. I have never had this problem since - it turns on every time. I'm not sure if this is your problem, but hope it helps.
