Strange voodoo electrics when I turn the lights on

You really couldn't make this up

Everything in the car is finally working spot on  (cabby gti with g60 conversion ) but when I turn the lights on,  the Rev counter, digital clock and the warning lights go off on the clocks.
Lights work fine though.

Where is best for me to start looking?

Just as a background, I picked up this car in bits and have only recently got round to putting it all together so I don't know if this issue is recent and related to wiring I have done or if it's from before I had the car

Bad Grounds.

Battery to frame/Frame to engine.
Bad ground on valve cover to the coil bracket.
Bad Ground on the cluster pin 2, or flaky 12v on pin 14

Is was a bad ground/negative behind the fuse box.
I must have missed it when fittingthe loom
On to the next item on the list of things to be fixed.

Cheers for your help,  I'm off to put the dashboard back in now 👍