There is a ground fault in one of your wire connections into your taillight. It is usually a connector for your rear light assembly. Remove the plugs from the tail light assembly starting with the side that blinkers flash fast when brakes are applied. You will find that the pin to your ground will have a blackish tint to the pin (on the brake light assembly). Clean this pin and the corresponding plug recepticle. I added a tiny piece of tin foil to the plug in the ground pin hole to ensure a tight connection. Plug in the unit while not installed and see if that eliminates the blinker lights from coming on on the dash when the brakes are applied and the blinker is on.
Check the left turn signal bulb. It is probably burnt out.
you need to change the flasher can for the signal lights
You can change the front turn signal, on your 2006 Pontiac Montana, by removing the retaining nuts. Remove the wiring harness. The turn signal will come free. Reverse the process to install the new turn signal.
Your Pontiac has a problem in the electronic block, which is controlling the turn signals, the tail and brake lights and etc. It shouldn't be a big deal.
Check the fuses
When the signal lights are not working on a car, there are a couple of things that could be going on. One could be that the bulb needs to be replaced. The other possibility is that the signal relay needs to be replaced.
the flasher located on the fuse block is more than likely burnt out
signal lights not working.
I had a car that had a failed combination switch (the turn signal, hi-low beam, parking lights switch). I could get the lights to come on if I set the switch to high beam, but not low. A new switch and, viola! Verrry expensive to have dealer repair. If this is your problem, hopefully you have a mechanic friend.
Check the fuse, check the bulbs, check the flasher relay, in that order.
Pulling the turn signal switch toward you should switch the lights to the low beams. If that doesn't work, you may have to replace the switch, which means replacing the entire turn signal stalk.
It's attached to the back of the hazard light button.