Rear turn signal not working, somtimes.
 

I have a 2003 Range Edge. About a month ago I noticed that my rear driver side turn signal was not working (the turn signal indicator was blinking and "clicking" more rapidly then normal). My first thought was that the bulb was out, so I went and bought a bulb got home went to fix it and it was working fine. I didnt do anything and it worked fine for a week or two and now it is not working again. The bulb comes on when I turn on the lights but it will not brighten when I use the turn signal or depress the brakes. Today I was going to get another bulb to replace the current one and when I was driving to the store it was working again, until I got home and decided not to go to the auto store then it decided not to work. It first started after we had a day or two of heavy rain, which could be a short of some kind but we havent had any rain since and it still works occasionally. What should I look for and how do I fix it. Help!