Wire center of one pole to the tail circuit fuse (originally to the HL switch for the tail/ park/ dash.
Wire the two outer terminals together and run that off to your tail/ park/ dash stuff. This means the switch is off centered, and those circuits will be live with switch either up or down
Now wire your HL power to the other center terminal. Wire the outgoing switch HL power TO ONLY ONE of the other terminals. Run that to your dimmer switch, run the high and low beam output to trigger two relays, and run their outputs to the lamps
This means the switch will be center off. One way (down?) with be tail/ park/ dash only. Other way (up?) will be those plus the headlights as well