I have a ford focus 2008 Zetec and the reverse light does not work. I've check the bulb and the fuse, and confirmed that when I put the car into reverse with the front wipers on, the back wiper does a single sweep (I've been told this is a good check for the reverse switch). 

The weirdest part about this is I tried adding a reverse camera using the Wires of the bulb (it was not working before I added the camera), but the camera actually works! As long as there is no bulb in the holder, as soon as a bulb makes contact with the bulb holder, the camera stops working too. The only thing I can think is the actual bulb holder is shorting something out when a bulb is plugged in. Anyone else experienced anything like this?

 

I took a short video of this phenomenon, uploaded it to youtube here: 

 

You most probably have a corroded / high resistance fault of the wires to the reverse light switch, or a corroded / high resistance connection to the Body Control Module.

The signal needed to tell the BCM that reverse has been selected is only a few milliamps, (and it then switches the rear wiper), the same is true of the load that the reverse camera takes.

The lamp takes nearly 2 Amps, so a poor connection would reduce the voltage to almost zero.

Get yourself a very low wattage bulb (2W) and try connecting that (a bit awkward but it can be done), and I expect you will find it lights up.

Most likely problem will be corrosion of the connections of the BCM.

It's worth replacing bulb which is cheap and easy to do. I pulled my focus apart trying to find a dodgy connection which turned out to be the bulb. I was thrown by the fact the bulb worked, but the BCM was detecting a fault and shutting the circuit down. When I checked the resistance on the bulb it was all over the place.

On my previous focus the reverse light did not work. But bulb was fine and voltmeter showed voltage at bulb holder. Taking plug off the switch, spraying wd40 in plug and putting the plug on and off a few times cured it.   Switch was on the gearbox.

On 3/31/2022 at 5:31 PM, unofix said:

You most probably have a corroded / high resistance fault of the wires to the reverse light switch, or a corroded / high resistance connection to the Body Control Module.

The signal needed to tell the BCM that reverse has been selected is only a few milliamps, (and it then switches the rear wiper), the same is true of the load that the reverse camera takes.

The lamp takes nearly 2 Amps, so a poor connection would reduce the voltage to almost zero.

Get yourself a very low wattage bulb (2W) and try connecting that (a bit awkward but it can be done), and I expect you will find it lights up.

Most likely problem will be corrosion of the connections of the BCM.

Cheers mate, can confirm the high resistance connection was on the reverse light switch. Thing was absolutely gummed with grease, suprised it was sending any current at all. Replaced it and it works a charm.