Headlight Toggle Switch?

Consider some options. Use relays, you need two. One for high beam, one for low. YOU NEED NOT "lose" the dimmer switch. the headlight power goes to the dimmer, which then switches between high and low

Consider a center off DPDT switch. That means "double pole double throw"

The "double pole" part essentially means it is two switches in one. "Center off" means just that, it has left, center, right or up, center down Double throw means it is "one" to separate circuits each side of center

HERE IS WHAT YOU DO.

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