What year?
What type of bulbs?
6v or 12v?
Original gauge wire?
If all original, and there was no relay originally, none is necessary. There has been no downside that I am aware of on my 6v 47-55 trucks with original wiring and bulbs.
However, some people add them, even in original systems.
If you are using higher-power bulbs than the originals, a relay might be beneficial (in time, material, and safety) if you have to re-wire.
