91 crewcab, relay on driver fender?

[v30crewcab]

working on a 91 crewcab 454 truck for a friend, it has single headlight, and we are putting a dual headlight front from a 89 suburban on it. it has a relay on the driver fender, near the hood spring, that the wires run into the lighting harness. the 89 suburban has the same relay, but it is wired into a different harness instead of the headlight harness. it is a 6.2 diesel suburban. what does it run, and why are they wired different? I've looked thru the factory 89 manual but can't seem to find any info on it.