A '74 will have a gorilla knob on the left of the dash and the salt shaker glow indicator right above the steering column, between the instruments. Gorilla knob starting is different, pull knob out partway and hold until you see a glow in the salt shaker; pull the remainder of the way to start.

Like Kerry said, these are series glow plugs ie: one element of the circuit fails and nothing works. Use standard troubleshooting to flash out the circuit, starting with the simplest thing first: the 'squiggly' buss connections between the glow plugs. In my book, wiring connections would be next in line to examine, then the fuse, then gorilla knob switch...

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