I have been working on a 1989 land rover defender 90. The brake lights work but not when I turn on the main lights. The tail lights do not work at all. When the main beam is on the brake lights do not come on. Anyone have any idea what I could be.? I was thinking maybe there is a relay not working right or the brake light switch.

 

Do you have a fully charged battery and clean/tight connection at terminals? Also, check ground connection at frame. Remove lens all the way around, front and back and check for a bulb that may be shorted. Do blinkers work?