your 81 came stock from the factory with 80006 pencil glow plugs, and a relay on the fender to control them.
I'd just change that if I were you, but the ford relay will work fine with the harness you have, just cut the end off, and link all the wires to the relay lug.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!