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I installed a relay that is operated by running the ground through a toggle. That relay controls the glow plug relay.
I did one of my trucks originally because I had a control go bad a fry a set of plugs. I also like it because when I run into a store and come out a couple of hours or ten minutes later the plugs don't need to cycle. Both my motors are in good shape. I can go to work on a summer day and go out at dinner time and they start just like you just shut them off.
You have to pay attention though and not let some fry your plugs for you.
You also want the control voltage for you add on relay to be on an ignition circuit. I also put a fuse in the voltage that triggers the glow plug relay. I also put a light in that circuit . If I see the light then I know the glow plug relay is getting voltage. That is my wait to start light. I use it to monitor the circuit I added.
This won't fix your starting problem unless you have a couple bad plugs and you let the ones that are good burn longer. Unless you have a controller that has failed.