What's wrong with the stop/turn signal on my 1999 F350?

I have a 1990 F350 super duty truck, the rear stop/turn light doesn’t work. It has voltage, light bulb and connector are new and connection is good, is some thing else I need check?

There’s voltage to the socket?

The first thing I’d do is switch bulbs, L to R, to see if the problem moves or stays put.

The good news is that it’s broken all the time, not intermittent. You know what the problem is and can reliably reproduce it; now all you need to do is isolate it.

(The second thing I’d do is run a temporary redundant ground. Voltage at the socket doesn’t do anything if it can’t get back home again.)

I did switched them, I used three bulbs, two new and one old, they all good at left side, but desen’t work on right side, the tail light on same bulb works normally, and that uses same groud wire on same bulb. The bulb is manufature sealed, and can’t be opened. I also using a DC 12V transformer jumbed the bulb terminal, the bulb is good. The only thing I’m thinking to do is jump the wire to the new socket’s terminal, and to see if it works? The voltage on stop/turn signal was 8, 6, 4, 3, 8V etc. Did not test the valtage on brake (stop).
Thank you all for the comments!

If the steering wheel is slung down as far as it can go, that can be a problem, try tilting the steering wheel up and see if it helps.

If 12 volts on the bench lights the bulb, then 12 volts on the car should light it. Right? You’d measured the voltage at the socket, and it is 12 volts, or 8 volts, or whatever. So remove the bulb, and run two jumper wires (one for power, one connected to the socket ground to bulb case). Does it light? If not, next measure the voltages with the bulb connected. If they are not the same or almost the same as without the bulb disconnected, that means there is an IR drop, the result of a high resistance, which means you’ve got a bad connection somewhere between the voltage source, and the bulb, or the ground source, and bulb ground. Because you are reporting inconsistent voltage readings, I expect you do in fact have a bad connection somewhere. Remember it can be in the ground as well as the power wire.