You must disconnect battery cable from its negative terminal on T100 models and turn off anti-theft audio system lockout if present. First delete airbag functions then extract the steering wheel before removing steering column trims. Locate and detach the socket from the combination switch which controls turn signals and headlights. When the switch is in OFF mode you should not get any electrical connection between Low or High beam but the ohmmeter should show continuity between terms 5, 12, and 13 when FLASH is selected. When the switch is in TAIL mode low beam and high beam terminals 10 and 11 must be linked while terminals 10 and 11 need to connect to terminals 5, 12, and 13 in FLASH mode. While in HEAD position the lamp system must provide connection at terminals 4-10-11 to terminals 6-13 for low beam and 4-10-11 to 5-13 for high beam plus links between terminals 4-10-11 and 5-12-13 in FLASH mode. The switch must get replaced when it fails to function.