In the past when I got the fast blinker is was because a bulb was blown. Now it is doing it and my rear left doesn't blink but putting in new bulb doesn't fix it. I also moved a working bulb in same assembly to it and it doesn't work. Any ideas on what else it could be?

 

Use a volt meter and check the supply and ground to the socket.

 

Probably rust/bad connection where bulb connects to socket.
Mine did the same thing.

 

The "board" that controls the lights in the housing could possibly be bad....ran into that problem on a Chebby a time or two.

 

Check the tail light assembly circuit board for corrosion.

MikeyB