That ice pick tester can be used as an on/off indicator for the low beam circuit. Just run a temporary wire from the low beam connector of the dimmer switch into the cab and connect it to the probe of the tester. Ground the pigtail and the tester will illuminate when the low beams are on.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!