I'm trying to tidy up the wiring from a recent overdrive installation and manual transmission conversion. I've scrounged a small relay box from a mid-2000's Ford that has room for two "mini" (typical automotive 5 pin) relays, and 4 "micro" (smaller rectangular 5 pin) relays.

I'm using the 240 Blue overdrive sequencing relay (on one of the "mini" slots) and one of the "micro" slots (fired from the indicator stalk switch). That all works great. No problems.

I'd like to go ahead and move the relay for the reverse lights into the box as well, but obviously the original 3-pin (SR803) bosch relay does not have a home. It seems like it shouldn't be to difficult to use a new "mini" or "micro" relay to do the same job, but I can't for the life of me get it working!

With the original relay in place, everything works as expected: when terminal 85 is grounded, current passes from 30/51 to 87 and illuminates the reverse lamps.

Simply replicating those terminal numbers on a new 5-pin relay, it does not seem to work. The reverse switch on the transmission is only a 1-wire switch. It seems like I need both a ground on pin 85 and a "trigger signal" on pin 86. But where would that trigger signal be coming from? I guess I'm confused about how the 3-pin relay works at all. There is only one +12v input, which is different from what I'm used to seeing.