If i recall, the tacho needs 12v, signal and 0v, ie earth
perhaps check all are as they should be first
ie, the 12v feed is actually 12v
0v/earth is a good earth
signal connections solid.
so many electrical problems turn out to be ‘dodgy earth’ that it’s always worth checking.
i’d probably try a temporary install with 12/0v from the battery and a short trigger wire to the tacho just to check !
steve