The turn signal unit is not really all that magical or complicated. It works as follows, assuming it is installed with the brake light included:
When the truck is going down the road and the turn signal is not engaged, the brake feed from the brake switch is connected to both sides at the back. When you turn the right signal on, it disconnects the brake feed to the right side and connects the blinker circuit. The blinker is a small interrupter with a time delay (so to speak). It is connected in series with the bulb. First the bulb turns on and then the blinker interrupts, and then closes and so on. It works the same for a left hand turn. The really nice feature for these new units is the four way flashers. They are a great safety add on.
