During a recent trip the glow plug indicator on my 82 300SD started cycling on and off while driving down the highway just as it would when the ignition is first turned on at startup.
The glow plug light comes on for apx 20 seconds then goes off for 30-40 seconds. This cycle continued until I stopped and removed the 80 amp bar fuse from the timer relay.
Does this indicate a bad glow plug or is the timer relay bad?

Glow system timer relay, or charging system problem is most likely fault.