Chevy Blazer

Ok, if your tail gate wiper works but not your washer, then you wither have a bad tailgate washer pump, or a break in the washer fluid hoses.
I would suggest taking the inner panel of the tail gate off and gaining access to the pump. See if it is working to start with. If not, check pin A on the plug for a good ground, and pin B for 12 volts when the washer switch is selected to wash. If you have both, and the pump is not running, replace it. It uses the same power to run the wiper in the wash mode as to run the washer pump in the wash mode, so if your wiper is wiping, your pump should be pumping. If your pump is working, then check the hoses from the resevoir to the pump, and from the pump to the spray nozzle for breaks or cloges.
Good luck.