hmm, i'm sorry buy i haven't had a single thing go wrong with the wiring on either of my porsche's! so have no idea which relay it would be.
i'd start by finding the fuse box and putting your hand on each relay in turn while someone turns the wipers on and off. if you can feel one of the relays operating, then there's power to the switch and relay. if not then it could be the fuse, switch, relay or wiring. it's worth pulling the fuse out and checking it with a multimeter, as i've had a couple that look ok, but have blown. then i'd check that there is power getting to the live side of the fuse. assuming you have power and the relay is clicking, then i'd check the voltage across the terminals on the back of the motor.
sorry i couldn't be more helpful!
jules.