If you haven't bought the relay yet I would try the lights without it. I'm pretty sure the 70 road lamp relays' only purpose in the circuit is to interface with the headlight delay and/or buzzer. It's been a while since I looked at the schematic and I don't have a 70 Plymouth electrical manual handy but I remember looking at it once and realizing that it was not 100 necessary for the lights to operate.

Mike Afflerbach