I wired up a SpecoMeter Tacho in my 1990 VN ecec. All after market tachos(external) should have these 4 wires. these are the 4 wires and where each of them go to the group of wires on the left hand side of the dash:
GREEN wire TO Brown wire(red stripe) RED wire TO Pink wire(black stripe) BLACK wire TO BOTTOM black wire WHITE wire TO Brown wire(white stripe)
you guys r lucky. coz it was a Bi*ch to find them out. i had to pull apart my mates dash and mine to look at how it was done. It works good as though.
A VN quote to Remember... 'THE MIGHTY SLEDGE HAMMER SOLVES ALL VN PROBLEMS' Anything can fit if its mashed into place...