well the one we tried was a face lift model with wires in bottom of bumper

so maybe that could be made a general rule

pre face lift fogs in the lamp units it will work

face lift fogs in the bottom of bumper it wont

well we cant be the only 2 people that have tried this but it does sound a good theory that pre facelift where the wires go to the headlight unit may already be live. I would guess they would have very little chance of getting earthed out if they are just in the multiplug, whereas like you say when they are down in the front bumper there is every chance of water ingress etc which would explain them being disabled at the ecu.