It sounds like the wire that powers the dash light is connected to the wrong terminal om the dimmer switch. If it's connected to the wire from the headlight switch to the dimmer, the light will stay on all the time, no matter which beam is selected.
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!