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Is the above with the engine running or not running?

it is probably poor connections somewhere and that can either be power or earth wires or both. Connections can be cleaned eg emery paper. Terminals can be cut off and replaced if really necessary though you do need a crimping tool and terminals.You can check connections with a multi/voltmeter or for power leads with a simple circuit tester which lights up if power is transmitted. Keep fingers well clear of pop ups. For checking processes you can check the net for info.

To me that sounds like a dodgy earth connection.

Here is a link to the wiring diagram for a 91 NA...

http://www.mediafire.com/?mn1mzmmbknw

The wiring diagram you are looking for is on page 3. You can try continuity testing the relevant wires using a multimeter.

You can test the resistance of earth connections by measuring the resistance of the joint (measuring between 'before the joint' and the chassis), ideal earth connections should have (almost) zero resistance

If it's a problem with your connectors, you can just cut them off and replace them with simple spade connectors.

My headlights have been connected just using spade connectors fo rthe last couple of years, and they work fine.

However, it does sound like an earth connection problem to me.