Hey guys.
My mates just installed a boost gauge on his car, but the instructions to get the gauge to light up on command from the head light switch are very vague.
The gauge has 3 wires, Positive, negative and earth (from memory)
Which wire goes to what so that the light on the gauge come on when either parkers or head lights are selected.

Cheers guys.

negative and earth are the same

should be 1 negative
and possible 2 positives

1 may be ign on power
the other should hook up to the interior light circut

I wouldn't hook it to the interior light circuit - you don't really need it to light up when you open the doors.
Hook it to the headlights or dash lights or cigarete lighter light - basically any light that comes on when the headlights are turned on.