Brake lights work without tail lights on, but not when they are

Hi guys,

One of my comrades noticed that my brake lights weren't working on the way back from a green laning venture yesterday. My tail lights are OK, but braking seems to make no difference to them. They work normally when the main lights are off. I swapped them for wipac after market ones recently, and I just swapped the cables over from the old ones, like to like - ie. red to red, green to green and black to black.

I've ruled out the switch, given that the lights come on OK when the other lights aren't on. Can anyone suggest where I should start looking at how to diagnose this, bearing in mind that my electrical skills are limited, but I do have a multi-meter and ratcheting crimpers etc

poor earths

dont know if this will help but i had same problem on mine when i put new wipac lights on and one of the wires that cross from right to left across the tub had frayed at the bullet connector when the wires were pulled out from the rear think it was a black one going up with a red and green from under the tub to inside behind the left hand side light if thats the earth "jamesmartin"sounds right.

Buy a power probe £18 on eBay it tests everything. I was haveing problems with my lights all sorted now thanks to using power probe

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Tom, check the earth wires at the back. Also do you have an earthing strap at the back of the chassis? All my rear electrics are earthed to the chassis on the near side - my brake lights stopped working when it broke.

Guys, thanks for the help. It turns out that the red and green wires are swapped on the Wipac replacement lights. When side/headlights aren't on, braking actually lights the side lights, and when the side/headlights are on, then both brake and taillight filaments are on together, hence the brakes not making any difference. Swapped the wires over, and it's all good

My britpart fuel tank sender unit had the wires the wrong way around as well, not sure why they don't use the original colours!