There is no "bulb" in the back. The LED is integrated into the tail lamp assembly. Replacing it means partially detaching the rear fascia.Usually, you have to disassemble the plastics in the rear hatch area then you will find a light assembly where you do a twist and the bulb holding assembly comes out. I haven't looked specifically on my volt to see how to get to the bulbs.
My guess too. It's the only incandescent signal "bulb".Your front right signal bulb is out.
You didn't mention front or rear so we have to guess.