First I had to figure out what cars it did work on (ST170 in my case), and then get the wiring diagrams and part numbers from a Ford parts guy in Europe. I first cross-checked the part numbers with Steve (FORDSVTPARTS) and he managed to find a US part number for the sensor. All I really need now is a spare harness so I can add two pins to the connector that's on my aux gauge cluster harness.
Your first step is to find out what the part number is for your cluster and what other cars it's used on. Then find which of those cars uses the indicator you want to enable. Get the wiring diagrams for that car and go from there.