John,
Remove the connector from the relay. The bottom of the relay should identify pin numbers 85, 86, 56, 56a and 56b. Identify the female connectors that correspond to pins 56 and 56b. Insert a jumper wire into the female connector for pin 56 and female connector for 56b. This will turn the wiper motor on.
I noticed that you have another post concerning blinker speeds. Did the problem with the blinker and wiper motor start at the same time? If so you may have an issue with the stalk switch. Both the blinker and wiper speed are controlled by this stalk.
Jeff C.
1970 280SL 4-speed