Why are high beams on when light switch is off?

If your sure that is what is is then it’s a cross connection in that circuit.

Site error.

Daytime running lights.

Are the taillights and instrument cluster lights on too? If not, daytime running lights. If so, is the high beam indicator on? If so, and you have automatic headlights, something is covering the light sensor on top of the dash, the sensor is dirty, or the sensor needs to be replaced. Jim

Might be a defect in the multi-switch on the steering column. That's a real possibility in ten year old Subs.