I don't plug my 97 7.3 in. It sometimes sites for a day or two. Fires rite up. If it does not when the temp is 0* or above you have a glow plug relay issue or a glow plug issue itself.
I have a TS 6 pos switch so I have the high idle built into that. I also put the ebpv on a manual switch so I control it. With that on and high idle, heat is made within less than 5 mins.