the easy way would be to tie into the tach or something and trigger at less than x (12.5?) volts. i've noticed the voltage at the cluster vs the battery is different tho. my shift lights/volt meter ties into the back of the tach and its usually .5 - 1 volt lower than the battery. so i'd say tying into your battery is the best bet. i'm sure theres stuff out there that would trigger at less than whatever voltage you want? installing the light should be easy enough.