Have you sorted this yet mate?

Just tried mine on the ecu feed pin 20 which comes direct from the battery through a relay switched by the ign.

STILL drops the power. Now I know the battery voltage is going to drop whatever whilst cranking, so I'm going to try straight to the battery and see if it still drops too much for the gauges on cranking.

only other thing it can be is a faulty map sensor on the gauge.