Just to the left of the oil filter are three things – an electrical plug/socket, a needle kinda thing, and a green slimy-looking deal with two wires coming out of it … it’s not on my '93, so my '93 harness has nothing for it … is this hurt me?
Fan switch.
On the 92+ cars, this coolant switch is located on the thermostat housing. As long as you use the right Thermostat housing cover and proper switch, you can leave that slimy old one alone.
I bow to you. 
I was going to guess that it was the FS this morning when you posted, but since you mentioned “'91 head”, I was confused since the switch is on the block.
Anyway, a picture is worth a thousand words 