Mine was to the point where it was just a quick flash when you'd turn the key on. The engine is healthy enough (despite burning some oil) that it's been starting, even without proper glow, just rough for those first 5-10 seconds.

It's worth it to replace the capacitors in your relay if you have the skills. Took less than 5 minutes to solder in the new caps and cost less than 50˘ in parts. Now I have a functional glow plug relay.

Of the 3 capacitors, one of the 100's measured .08µF, the other was 0.00. The 7µF measured .18µF. They were definitely toast. Seems to be a common occurrence with electronics on 80s Benzes.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]