The white wire is for the rev counter. I`ve converted my Mk1 aswell, from a 2 dial to a 6 dail dash. The white wire is connected together, with grey connectors. You must sepperate it and then he wire of the dash for the rev counter goes in between, so the rev counter sit in the circuit.

White wire`s on dash, if you look good, with the grey connectors (male and female):



Not all Mk1 standards have that white wire ! Must of them do, but not all of them. But you say you`ve got a white wire, so should be good

Hope you understand me, my english isn`t that great..
Dutch guy