Honda Pioneer SXS500M 2019 Owner's Manual page 170

Brakes
Brake Light Switch Adjustment
Refer to Safety Precautions on page 91.
Periodically check the operation of the brake light switch. It is
located under the front hood (page 105), just above -- and slightly
behind -- the brake fluid reservoir.
The brake light should turn on just before the brake works.
Adjustment is done by holding the switch body and turning the
adjusting nut. Turn the nut in direction (A) if the switch operates too
late, and in direction (B) if the switch operates too soon.
UNDER FRONT HOOD
LEFT SIDE
A
B
brake light switch
160
Servicing Your Honda