Mechanic Q: my rear turn signals aren't working. I I checked the Turn?

my rear turn signals aren't working. I I checked the Turn Signal fuse (#6) & replaced even though it seemed fine. That didn't help. Out of the 3 bulbs in the rear tail light only the top ones are not working at all. I replaced both but that hasn't helped. Front turn signals work but click very fast instead of the normal slow click. What do I do???