Has anyone had this problem? The brake lights work on my 1996 90 BUT only if the rear lights are on.....

Bad earth. 150 times out of 100 it is always a bad earth when talking electronics and Defenders...

As above it is always a bad earth.

When you switch the side lights on do the brake lights come on too or do you have to press the brake pedal?

nobber said:

When you switch the side lights on do the brake lights come on too or do you have to press the brake pedal?

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Press the brake pedal

GG999 said:

Press the brake pedal

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Side lamps are powering up the brake lamps, could be corroded bulb holders, you need to pull all four corners apart and have a look, do you have a probe and multimeter?

also, study the wiring diagram, follow the wiring through and test at every wire break, bulb - switch - fuse.
Im pretty sure the side lamps are on one circuit but without a diagram I cant be sure. Also, check all the earths while you work through it.
If you follow it through methodically, starting at the bulbs and testing each circuit you will find the fault, if you dont have a probe and meter you are basically looking for visible corrosion , chaffed wiring , that sort of thing but with no test equipment
and a harness running through the chassis a problem with the actual wiring wont be easy to find, not impossible, but not easy.