If the ecu outputs a negative down this wire then terminal 85 of the relay, if it outputs a live down this wire then terminal 86.
Relay is wired,
85 = earth
86 = live
87 = output(live from 30 once "on")
It doesn't matter which way around 85 and 86 are so long as one is earth and the other live.
tabetha
30 = input from battery via fuse, live input