Funny thing, Glow plug light on my TC33da started acting up the other day. First checked the ignition switch. Opened it up and found no fault, regreased it with dielectric grease. Then checked the connections with the relay. I could get it to light intermittently wiggling the connector. It didn't look burnt, but I loaded it with grease and wiggled a bit and it seems to work solidly again. Time will tell, but That would be my first check.