Have you tried connecting it directly to see if it works? If it's TTL logic all it needs is current, the 5ma might be enough, might not. With the 12 volt line shorted to the ECU (so the AC is on) what voltage do you get at that pin? Just use your meter in voltage mode, and attach the black wire to the car frame, see what voltage you get at the ECU pin while it's on.
Sorry I'm asking so many questions I'm not very familiar with the type's of inputs that are on car ECU units so I gotta cover my bases to try to understand what to recommend.