Left rear turn signal not working only

I have had a few people look at my left rear turn signal light that will not work. All the other parts to the taillight and brake light works. I have even replaced with a new bulb and it didn't work. I can't figure out how to fix it? Do you know what it could be and how to fix it? Someone said maybe buy a multi-functional turn signal switch, but if all of my other signals and lights all work right but that left rear signal light would I still need to buy a multi-functional switch? Also, the people said something about it wasn't getting grounded right maybe or something what does that mean and how can I fix that?