My husband can start it now, by direct connect from battery to the ribbon like part on the glow plug controller. It directly heats the glow plugs. We shut the engine off after only short time. All this is manually done, running a hot wire between the ribbon and the battery. We want to put a switch on it but not sure how long to hold the switch. By touch, it takes 25 seconds for the ribbon to be uncomfortably hot. We're wondering what type push button switch would be sturdy enough. One, we bought has tiny wires and we don't think the wire is a good gauge for our use. enough.
You should excite the relay, that does the same thing...unless the relay is the trouble. Then you wouldn't need a heavy switch, because the relay takes all of the amperage, and the switch only pulls the coil of the relay in. There is also a fusible link, right off of the starter relay (on the passenger fender well) Sometimes they will burn mostly through, but not burn the insulation. It will look perfectly normal, and there will be voltage to the hot side of the relay until you put a load on it, then it all goes away, because there isn't enough conductor to carry that kind of current. that one was a head scratcher for me for about an hour when I first got the truck I have now. It sounds like everything works beyond the "ribbon", so the trouble is before it gets there. My next step would be to check the fusible link, mostly just grab hold of both ends, and pull firmly. If it comes apart, it wasn't any good anyway. Also with an assistant, check the voltage on GP side of the relay when it cycles on. It should be somewhere around battery voltage, if it drops to nothing, definitely check the fusible link. Just follow the wire harness back to the relay on the fender well.
We had the wiring harness checked out because we were worried about the fusible links. They said they were all fine. We ran a wire directly into the battery side of the controller and no power goes past the "ribbon". We replaced the gp controller, no help.