We have an F-550 that won't start in cold weather. cold. It starts fine with the heater plugged in
I have checked the glow plug solenoid, the harness to the valve cover and the resistance from the cover connector to ground. I have .6-.8 ohms on my pins.
Is there anything else I should look for in this system.

 

You might consider starting at the glow plugs and see if any power is getting to them when required. If not, work your way back through the wiring to determine where the power is lost. If it is available at the relay, the problem has to be between the glow plug relay and the glow plugs.

Considering the age of the vehicle , there is a possibility of the glow plugs themselves could be the problem.