Easy. Your Volvo 240 owner manual or a Bentley, Haynes, or Volvo green OEM factory shop service manual helps.

The clear lens can separate from the reflector body on the big, square 1986 and later 240 headlights.

At the front center bottom edge of the clear head light lens, gently pull forward and up. You may need to remove the grille to see the clear headlight lens pulling away from the reflector body.

Separation of the the reflector body from the clear head light lens can allow moisture and biomatter intrusion into the head light reflector chamber.

You can search the brickboard for threads containing various procedures and materials to clean and re-seal the headlight lens and the reflector body for 1986 big square headlight 240s.

You usually want to replace head light in pairs or at least use the same make and model bulb.

It can hurt to check the wiring, verify headlight and turn signal wire harness an grounds, and the like.

cheers,

Egged and Nawgggged (as in eggnog).
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Eggnog.