Is there a way to test the under dash flasher relay. I have a rear passenger
side flash not working. I checked for voltage (12V) at its socket and got
no voltage, nothing.


I would prefer not to buy a new relay and find out that is not the problem.

 

If you mean the thing that makes the bulb blink, if the remaining lights in the truck flash, it's not the blinker. If you have one socket with no power then you need to start tracing that wire and see if there is a break or short somewhere along the way.

 

WoW ! tracing the electric lines and ground from the fuse box to the
right rear turn sign may be too much for me. (age)


Anyway, thanks for your help.

 

if all the other lights work, I think I would be checking for a good ground at that rear corner. Maybe the ground wire is corroded / broke ....... normally the bulb or the ground..not the power line.

 

try the fuses first.

 

If all the other lights work but that one it won't be fuses. Each individual socket won't have it's own fuse.
Checking the ground is a very good idea and a great place to start assuming the bulb is good.