Replace the bulb. A filament can break "in the middle" and then move around - connecting, disconnecting, repeat. Replacing the rear bulbs on the GC is quite easy. Buy (2) good quality bulbs (I use Sylvania) and replace both rears. The factory bulbs are quite low quality. I did this on mine when the right rear went out after 6 months and 5000 miles. Not a big deal. I have had to do this on every new vehicle purchased in the last 10 years or so. Common problem no matter the make or model with the standard taillight bulbs.