I just did this to my 87 installing a inline frame mounted Walboro pump retaining the factory oil pressure wiring and shutoff.
I pulled the switch then disconnected the harness at the back and stuffed a test light into the terminal turned the key on and the pump power will light up for 2 seconds when you get the correct one. Should also light up if you crank the truck 3 to 5 times showing you that the oil pressure circuit is working properly.
I cut the wire a bit back from the connection soldering the old to new wire and then taped up everything before popping the old switch in place so there would not be a hole in the dash. Then I ran the new 10 gauge wire to a relay near the pump to lessen strain on the factory wiring.
Your tank sending unit wire is the pink one located down by the switcher. The pink and white goes to a tank and the pink and black goes to the other tank. But the solid pink one is the one that goes to your gauge and is what you want to hook up to your new tanks sending unit.
Here is a wiring diagram for the 2 tank switch setup.
