you checked the inertia switch mate? sits down on the A pillar behind the little cover in the passenger side footwell, theres sometimes a hole in the cover you can reset it through. worth checking!

if not then yeah you can wire it like you said, ive wired mine like that in my zetec conversion.

you want a normal switching relay with the switchable live coming from the igntiion, then you need to wire the earth from the relay into the earth that goes into the ecu. if you look at the engine loom, between the engine and ECU there will be a bunch of 7 wires that come away from the main engine loom and plug into the car loom, the brown wire in this group is the one you want!

make sure you fuse it properly and wire it in through the inertia switch!