I have a 1984 E150.
Recently the left rear running lite is much brighter than the right one.
When the brakes are applied the right rear bulb goes out completely.
Have changed both bulbs.
Turn signal work but 4 way flashers alternate on and off.
Any help would be appreciated.
Thanks

 

I suspect the right rear light bulb socket has lost its ground and is using the the L/R stop lite bulb filament for it's ground. You will probably have to replace the R/S bulb socket.
Here is how you can prove that, take a piece of wire and connect one end to a good ground then touch the R/S bulb base with the grounded wire, you should see the difference.