if you mean the Toyota hilux type and usually most other vehicals the main beam is on the arm that you turn your lights on with all you have to do is gently click it towards you and the blue light on dash should show which means your main beam is on click again it should return to low beam with blue light out hope this helps you out.
check the fuses then check high/low beam switch
The high beam switch is on the reverse of the turn signal switch arm
Try the floor to the front left. There should be a switch you can press with your foot.
you need to replace the high/low beam switch
bad light switch.
Check fuses and high/low beam switch.
It is on the signal light switch. Pulling it toward you will change from High to Low beam lights and vice-versa 7T77T7
The last vehicle to have it on the floor was the 1991 Ford F-Series pickup.
Probably the high beam switch in the steering column is bad.
you will probably have to replace the headlight switch.
I own a 1991 F-150 and had same problem. Check the high beam (floor) switch. That was my problem, I normally kick the switch.
The dimmer switch if for the instrument panel lights. The high and low beam switch is for the headlights. On most older cars the high and low beam switch was on the floor. Most cars have it on the turn signal lever now.