If the windshield washers don't work, the first thing to check is whether there is fluid in the washer tank, and that the washer pump runs. The tank and pump are located in the boot, just behind the front of the car. Follow the little plastic hoses and you will find it. If it doesn't run, make sure it is getting current when the wiper stalk is pulled to the "wash" position. If it isn't getting current, trace out the wiring to ensure continuity. If the wiring is good, you may have a bad wiper/washer switch.
If the pump has current but still won't run, disconnect the hoses and electrical connectors, and take out the motor, which is held in place by two small rubber tabs-it just pulls out. Disassemble the pump section on the bottom of the motor, clean it well, reassemble and test. Sometimes connecting the power to the motor backwards will make it break free and run (it won't hurt the motor). When you reinstall the motor, be sure to connect the brown wire to the motor negative terminal (brown is always used for ground in Porsche). If the motor still won't run, you need a new pump. This pump is used for the windshield and headlight washers on the 1978-1983 911SC. The headlight washer has a separate relay, hidden in the dash behind the instruments.
If the pump runs but the washers won't squirt, you have a blockage somewhere in the plastic lines between the washer fluid tank and the squirters. The washer nozzles themselves may be blocked with wax residue. You can clean this out with a straight pin, and you can also use the pin to aim the nozzles just where you want them to squirt. Blockages are common, and it is just a matter of making sure each section of the lines is clear, starting at the tank. You can disconnect each section and blow it out with shop air, or with the washer pump.
A prime location for blockages is the T-connectors, and the tiny check valves at the base of each washer nozzle. The check valves hold pressure in the washer system, so the washers can squirt as soon as the pump is activated, instead of having to pump fluid all the way up to the nozzles each time. To get to the check valves, pull back the carpet in the boot and remove the fiberboard dash cover. After removing the check valve, you should be able to blow through it in one direction, but not in the other. If the check valves are plugged or stuck open, you may be able to free them up by soaking them in white vinegar. If they still won't function as described, replace them.
This same troubleshooting method applies to the headlight washers. Keep in mind that they will function only when the headlights are turned on, and they spray a fixed length of time each time the switch is pressed. The reservoir for both washers is in the LF fender, and the filler is located next to the fuel filler. If you have the "high intensity" headlight washers, they have their own pump. To get to the fluid supply lines and replace broken or defective nozzles for the headlight washers, it is necessary to remove the front bumper.