The brightness of the interior lights is changed by adjusting the position of the dimmer switch thumbwheel on the headlamp switch. The thumbwheel controls a rheostat. The rheostat changes circuit resistance, so it is capable of providing a variable voltage to the system lamps and components. The lamps respond directly to the voltage and are therefore capable of producing variable levels of illumination.

The PARK LPS fuse supplies voltage to the headlamp switch through CKT 240 (ORN). With the headlamp switch in either the park lamps or the headlamps position, voltage is available to the rheostat. The rheostat provides voltage through CKT 44 (DK GRN) to the ILLUM fuse. The ILLUM fuse provides voltage to system components through CKT 8 (GRY).

The dimmer switch does not directly power the radio, PRNDL, or odometer illumination, but the dimmer switch voltage is used as a signal. With no voltage on CKT 8 (GRY), the radio, PRNDL indicator and odometer are at daytime brightness. If there is voltage on CKT 8 (GRY), then these components are dimmed in proportion to the setting of the dimmer switch.

In some circumstances it may be desirable to have the radio, PRNDL indicator, and odometer illuminated at full daytime intensity even though the exterior lights are on. If the IP dimmer switch thumbwheel is placed in the parade mode detent position, the switch overrides the interior lights dimming for the radio and PRNDL indicator, and odometer. With the switch in the parade mode, voltage from the PARK LPS fuse is available to CKT 1382 (PPL/WHT).