tach install question

i am installing an autometer tach in my 91 pickup. i have everything working now except the light on it. i am supposed to hook it up the headlight switch or a wire that turns on when i turn the lights on. can someone please tell me what wire or were to hook it so that the light will now come on when i turn the lights on. thanks

The dash light dimmer switch is the easiest place to get to!!