Indicator fault

Got an issue with my passenger side indicator on my 90. It won’t work when I have the headlights on. Will work when the sidelights are on or lights are completely turned off. Hazards work with full lights on. Weird . I am terrible at wiring and fault finding in electrics, any help would be much appreciated. Thanks

It's an earth its always an earth, or should I say lack of

Start at the headlight/indicator and check your way back. make sure the bulb holder is clean to start with.
Do you have a DVM or a test screwdriver (the 1 with the little light in)? Cos you should have.
J

good old Lucas wiring

fiddle with the connectors, earths and fuses, then swear a lot & get a tester as suggested by @marjon


Rich.