Dip beam problem

I have an interesting problem with dip beam on my D4 commercial.
Drivers side dip beam went off yesterday. I assumed a failed bulb. Passenger side went off today. Two new bulbs fitted and still only got passenger side back.
Further investigation show a good 12.8v at the terminals with no bulb attached, and the bulb tests ok when fitted to the other side. When the bulb is fitted there is no voltage showing on my Meter, but when the bulb is removed the voltage reappears

Any help would be greatly appreciated

Many thanks

Rob

Two choices:

1. Bad earth... go along the earth wire if you can or rewire an earth from a good point

2. bad contact in the lamp's connector or somewhere in other connector on the circuit or chaffed wiring... that's harder to trace

on bad contact the voltage shows as the voltmeter puts no load on the circuit but when you put load the voltage drops