For changing headlights from high beam to low beam, a dimmer switch is mounted on the floorboard or on the steering column (turn signal lever).
Two types of dimmer switches. Courtesy of Ford Motor Company.
To diagnose a suspected malfunctioning dimmer switch, a fused jumper wire can be used to bypass the switch to either low or high beams. The dimmer switch is typically connected in series with the headlight circuit, so when the switch is bypassed, the low or high beams will go on unless something is wrong in the circuit.
On dimmer switches that are part of the multifunction switch, it may be possible to test the switch without removing it from the steering column. Normally there is a multi-connector at the base of the column that can be used for testing the switch. If not, it may be necessary to remove the column covers to gain access. Refer to specific vehicle manufacturer guidelines for this procedure to prevent damaging access covers or connectors.