Headlights problem

Just noticed an odd problem on my 79 TA, anyone know what the cause is?
Pull the headlight knob out once and the 2 dipped beams come on as they should. Pull it out a notch again and the headlights come on as they should.
Now if i put the full beam on so all 4 lights are on one of the headlights dips down to dipped beam level. Turn full beam off and it goes back to headlights ok.
To explain that a little better if you're looking at the front of the car with full beam turned on. From left to right you've got
On Full, On Full, ---- On Full, Dipped
Any ideas?
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Bad ground on the side with the dim light.
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