glow plug light remains on
 

On my 79' 300TD, when I go tostart it up, I noticed that the glow plug light no longer turns itself off when its ready, ie hot. It just remains on. So, I've been waiting the 30-40 seconds it usually takes, and it fires up like it always has, then it turns off. I would like to fix it though. Suggestions? My guess is the relay, but then I've never dealt with the glow plug system. Thanks.