hey i recently fitted a tacho tom my micra, and it was quite simple there is normaly 4 wires which are
nutral - wire up to any bolt in the car ( natural earth )
negative - wire up to the negative will coming from your coil
positive - i wired mine up to my side lights
backlight positive - this just lights the revcounter up on a nite also wire up to side lights
hope that helps
benben