No Electrical power battteries 100%???

[hatzie]

The glow plug contactor isn't exactly the same as a starter relay. As you mentioned... starter contactors generally ground through the body of the contactor. The big round GM glow plug contactor on the early 6.2 & 350 Olds diesels were isolated from the vehicle and had both low current power and ground connections...
The original wiring to the big glow plug contactor switched the ground. If you intend to use this contactor in manual mode I'd put a starter push button on the knee bar of the dash and switch the ground to the GM glow contactor. AutoZone has the big rubber top tractor starter buttons for some reason.
My 1967 IH Farmall 656 uses a push button to run the glow plugs and it works quite well with modern self-limiting PTC (Positive Temperature Coefficient) glow plugs similar in function to the AC60G. 51 years old and she starts just fine at -30°F.

Are you sure you want to have manual glow plugs on a truck your teen or young twenties daughter is driving? Obviously depends on the girl but I wouldn't let my nieces or even my nephews drive with manual glow plugs. Attention span of youngsters and all.

Diarc posted copies of some of my manual scans... You can get them here as well along with quite a few of the GM Service Training Group textbooks. Look in my signature.

That's a very good explanation of what's happening with a snubber... No question about what's happening when you can visualize the effect on a scope.

I neglected to add that you need to reverse bias snubber diodes. Otherwise it's just a dead short in the circuit.
Reverse biasing a diode means that you hook the Cathode end to positive and the Anode to ground.
The Cathode end of a regular diode is the end with a stripe.