I have a 1999 NH LX565. The engine preheater has quit working. When I turn the key on, it counts down same as always, but does not heat. I'm thinking it's either the relay switch or glow plugs (although I can't imagine all the glow plugs are bad). There is also a 50A circuit breaker. Anyone had any experience with this system? Also, how do I test a glow plug to see if it's bad? Thanks in advance for your help.