Rev counter dies when lights are on !

As per title, all is well with the Rev counter until I switch the lights on, when the Rev counter needle just drops. Seems like a dodgy earth but all seems connected.
Ive read that it could be a disconnected earth strap on the rocker cover, but the cars had a 20vt conversion. It used to work with the lights on. The cars an ‘83 1.3 Driver with an AMK engine conversion.
Any suggestions would be gratefully received.

replace your battery to frame and frame to engine grounds if they aren't new.  

Replace the ground from the valve cover to the coil bracket.

On your cluster you want to remove it.
You want to splice a new ground from the  frame to the brown wire connector side.

Verify that you have 12V on the black wire.

Take the connector shroud off the cluster to expose the mylar, carefully remove the mylar off the plastic tongue, on the pin side of the plastic tongue place a single strip of electrical tape on the tongue, replace the mylar, and then the shroud. Replace the cluster and reconnect and then give it a try.

if its standard mk1 dash and wirign loom, the clo0cks earth in with the main dash earth to the white 4 way spade block on the headlight loom. look for any spare earths flapping about with brown or black housings.

make sure the brown on the white D plug is earthed to the block above, as well as any thick earths from the dash and rear looms. thinner earths without spade housings go to the claw points above the fusebox

also note out the back of the white 4 way earth block is a thick brown with spade housing, this needs to go to the earth claw above the fuserbox as well, it provides the earth for them they do NOT earth to the shell