My 68 f250 has a tail light issue
right rear light, when engaged(turn right) start to flash both sides. When left rear light is engaged(turn left) nothing?
ideas???

 

First place to go are the grounds. Next is the turn signal switch inside the column.

 

This can also happen if bulbs are not fully inserted or if the wrong bulb type is used - single versus double contact. The wrong bulbs can be made to fit but won't work as intended. But, Alex is right - first start with the grounds. When a ground is bad or missing the circuit will find a way to complete - often through another light in the system.

 

thanks on the info... great insight on builds.
where are these grounds?

 

Anywhere there's a black wire.