Find your Intake Air Temp sensor. It’s the sensor just to the rear of the throttle body, has 2 wires, and screws into the intake manifold.
Where the IAT connector plugs into the harness you will see that one of the wires on the harness side is brown with a white stripe. Follow the brown with white stripe wire back into the harness.
You’ll have to open up the split-loom plastic sheathing to follow it. It will come to a splice with 2 other brown with white wires with duct tape over them. They’re from the TPS and the CTS. The 3 wires will be spliced to a single wire headed toward the C101 connector if you have an ’87 or ’88. If you have an ’89 or ’90, you do not have the C101 bulkhead connector.
Now go to the MAP sensor.
Follow the brown with white wire into the harness from there. You will find a splice with 2 more brown with white wires with duct tape over them. At the splice you will find the 3 wires connected to a single brown with white wire going toward the C101, or just along the firewall towards the engine if you have an ’89 or ’90. On an ’89 or ’90 you will find 4 wires going to 1 wire. Along with the MAP sensor that you traced, they are the ECU sensor ground port and the diagnostic connector on the passenger inner fender.
You now have 2 sets of 3 brown with white wires, one near the firewall and one near the engine. If it’s an ’89 or ’90, you have 4 wires going to 1 wire.
Cut the splices out of each set of wires eliminating not only the crappy factory splices, but also the single wire between them. Bring both sets of 3 wires together. Solder the 2 sets of wires together and insulate them properly with tape or shrink tubing.
Zip-tie up your new sensor loom to allow for engine movement. I prefer to cover it with some new split-loom or wrap it neatly with electrical tape when done.