I wired mine in. Mine was an Astra engine, might be different on Cav one but if you look near the ECU there were 2 spare wires on the loom going to a black plug, brown/white and brown/yellow. I cut the plug off this and connected the wires to the corresponding coloured wires on the diagnostic plug. Then just connect the red and brown from diag plug to live and earth.
Useful to have so you can read fault codes if the ECU light comes on.