It's got a series glow system if it is the original system. One bad glow plug causes the whole system to fail. It also has a fuse on the firewall at the back of the engine on the firewall.
The best way to deal with this system is to replace the series glow plugs with the retrofit parallel plugs. You can remove the heavy squiggly wires, remove the ground wire from the first glow plug at the front of the engine, put in the retrofit plugs, hook glow plug 5 up to the hot wire from the firewall and put jumper wires between each plug.
Searching on here should give you the part number for the retrofit parallel pencil plugs.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13