How does a glow plug work?

[MaxLee]

So at the beginning of the season noticed tractor (brandson 4720H) was getting harder to start on cool mornings. To a point when it just didn't. Just rolled over and over. We had a break in the temp a little so I picked up 4 glow plugs Thinking I could replace them all or what ever was required. . Since I had to remove the metal strip running to each plug and just replaced the first 3 . Last one is very difficult to get to.. 30 minutes later I'm all done . Fires right up in 5 seconds. Next day 5 seconds . Next day 5 seconds. Today no go. Glow plug light on dash is not even lit. Checked all fuses all good. So what is the power that a glow plug operates on . 12V?? Is there any other devices or moduals if the lineto operate a glow plug?
All I can say is the glow plug wire is about 3 " long before it enters the harness. This is as far as I got so far. Storm coming in 4 days. Needs to be running.

I had a wire broke coming out of the glow plug heater control. Mine wouldn't light on dash at all ( could crack it over for ever)unless I turned key all the way just before the start position. It would come on and a would count o 15 then turn to start it. I thought it was module and when I decide to see what part I need I found the wire.

 

[MaxLee]

So without a drawing should I be looking inside console under the dash or under the hood . Maybe cut open the harness and start following?

find the module then do ohms test and see if any wire broke, check voltage in and out. it seem more involved then it is. basic relay low voltage stuff , make sure key is on when checking voltage , actually I would start at the glow plugs see if voltage there then work back, could even be the key switch , sorry about confusion, thinking while I am typing LOL