marlin758, i have the scanner, thanks. it actually has an option to switch off cold and/or hot checks on each light (see the Light Coding screen
here). that didn't work, which makes me think something on the lines of what Sanj is saying.
Sanj, the symptom is as below:
Scenario 1. engine not running, key in position 2, put gear gear into reverse-> relay engages, camera works perfectly.
Scenario 2. engine running (key in position 2), put gear into reverse-> relay starts making a continuous pulsating 'TRRRRRR' noise! audibly it must be like 10htz or so.. the camera still works because the frequency is enough to flip the video module to show the camera feed. but occasionally the screen flickers.
the bulbs have a hot check and a cold check. so i assumed this to be the hot check.
in fact i did a further test: while in Scenario 2 above, i touched a direct +12v onto the reverse light pin on the relay (through a 5A fuse just in case). and sure enough the relay stops pulsating and sure enough, a 'Check Reverse Lights' comes up when the key is later turned off. this is why i assumed it must be the hot check.
Scenario 2 is difficult to test alone: the engine running, gear in reverse, and I have to be standing behind the car meddling inside the boot!! literally asking the car to run me over!! :'(
ps
also my relay is next to the video module, so it can only be observed/heard with the the spare and compressor off, because otherwise the sound is muffled and cannot hear it from inside the car.