How do you test a fuel pump and relay for a 1991 vw cabriolet?

To test the relay pull it out of the relay/fuse board. Use a fused jumper to give 12v to pin 85 with another jumper from pin 86 to ground. The relay should click. An ohm meter on pins 30 and 87 should show infinite resistance ( open circuit ) when there is no power to pin 85 and should show low resistance ( closed circuit ) when there is power to pin 85. If that checks out, turn your key to the on position ( not start) and use your fused jumper between the plugs where pin 30 and pin 87 plugged in to the board. Be careful when figuring out wich pins went in wich plugs so you don't jump the wrong ones. When you do this the pump should start and you will be able to hear it hum. Don't run it for too long, just long enough to check that it comes on. If it doesn't come on use a volt meter to make sure you have battery power between the plug where pin 30 goes and ground. If not you may have a blown fuse or bad wiring somewhere. If you have 12v at pin 30 test the wiring between the fuse/relay board and the pump. Disconect the wires to the pump and use an ohm meter between the plug on the relay/fuse board where pin 87 plugged in and ground. You will read infinite resistance ( open circuit ) if there is no short.