For 1990-2009 cars

Voltage applies at all times through the TAIL fuse to the tail relay. Turning the turn signal/headlamp switch to the PARK or the HEAD position provides a ground path to the coil side of the relay to G202. The ground path energizes the coil of the relay. This closes the contacts in the relay. The closed contacts apply voltage to the illumination controller and all the illumination lamps. The illumination lamps are ground through the illumination controller. Since the illumination controller is ground to G200, the lamps illuminate when voltage applies.

The illumination controller controls the brilliance of the lamps by controlling the amount of current from the lamps to ground. Turn the thumbwheel for more or less current. Increasing the amount of current increases the brightness of the lamps. Decreasing the amount of current decreases the brightness of the lamps.