A blinking headache...

As mentioned, GM combines the stop and turn signals into a single lead for use with the factory taillight, using a standard, type 1157, dual filament lamp for run/turn/stop.

The first issue was isolating the signal for the turn signal.  I decided to tap into the front turn signals.  This involved running two conductors from the rear bumper, street-side, where the taillight wiring junction sits behind the backup light, up to the front, under the hood, near the left turn signal.  At that location, both the left and right turn signal leads are available in the factory wiring harness.

Now, with the signal in hand, the next issue was the flasher itself.  The flasher senses current flow, which controls the speed, or frequency of the flash.  Since an LED light draws almost no current, the flasher behaves as though taillight is burnt out or missing, which means it flashes at a rapid rate.  That could be a cosmetic issue that might be ignored.  It works, its just to fast.  In our case, the turn signal is actually a sequential flasher with five arrows.  At the fast flash rate, only the first three arrows would fire before the lamp goes "off."

This is a common scenario in the LED aftermarket.  There are two solutions.  Perhaps the better solution is to replace the flasher unit.  I didn't explore this option, because it is vehicle/model specific, and I didn't feel like going the Mr. Goodwrench route.  The second solution is generic, simply trick the system to think there is a standard bulb in the circuit.

You could actually wire a socket/bulb in parallel with the turn signal, but then you would have to replace the bulb periodically.  Alternatively, a resistor will perform the same function and never need replacement.  The resistors used are six ohm, 50 watt, and available from places that sell LED vehicle lights.  I got ours from SuperBrightLEDs.com.

The resistors are connected on the back of the turn signals on the side of the vehicle, next to the wheel wells.