Mine works fine....
Can you hear the motor trying to work, if so, there might just be some sort of blockage thats not letting the fliud come through. If you can't even hear the motor trying to work then maybe there's a problem with the wiring or something.

 

mine works...i rigged mine up one day to spray people behind me...really pisses them off...

 

Yes. I can hear the motor. So my guess (and ofcourse your's too) is that there is some blockage, since I am taking my car out after a month. Any ideas, how to check/clear the blockage?

 

First I'd ream out the rear sprayer nozzle with a straight pin. The most inconsequential amount of road muck can plug that nozzle up.

If that doesn't work, then it comes down to partially removing the headliner to access the supply hose from underneath. Not a fun job by any means.

OTOH, if you're in the snow belt, this could just as easily be ice in the lines (unless the garage for your car is heated).