Relays are easy to test, provided you understand how they work. Is the windscreen relay similar to the standard Bosch units for the lights and load-shed? If so, unplug the relay. Relays have a control side and a controlled side. The control side (small load) kicks the relay over to run the controlled side (big load). Put your circuit tester on the control side first and hit the switch. If it lights up, then you know the control side is working. If it doesn't, you're open either before or after the relay. Run the positive to ground and if it lights up then it's after the relay. Now, plug the relay back in and hit the switch. The relay should click. If it doesn't, then you know it's a bad relay. If it does, check the leads on the controlled side. If your getting no continuity, then your relay is bad or you have a bad connection somewhere before the relay. If you have power at the relay and after but no joy at the windscreen, then you're open somewhere between the relay and screen. If it's not a standard Bosch relay, the same as above applies but you'll have to determine which lead does what as far as the control/controlled side. Hope this helps!