I would get a voltage meter or test light and put it on the the rail that all the glow plugs are on. If you are getting power there and the light on the dash is working, the relay is most likely fine. I’d suspect that you have one or more glow plugs going out. You can test it by taking that glow plug rail off and putting power to each glow plug individually. It should spark when you touch it. If not, it’s most likely bad. A more detailed test is to remove each glow plug and bench test it and see if it gets red hot. It’s a pretty simple system actually. Basically like a small tractor motor

Check all the grounds and fuses. If you apply power to one of the glow plugs, you should be able to see the light on the dash work. If you have power to the relay and the glow plug light comes on when you supply power to glow plug, most likely the relay