You're getting voltage at the glow plug harness connector on the relay but presumably nothing at the glow plugs because the glow light isn't working? Very odd. I suppose the harness can be so brittle that what movement it went through as you did this job broke all 5 wires. That can't be the prime suspect.

Did you do a resistance check of the new glow plugs at the relay connector? Did you check each plug before installing?

Did you check for voltage at the glow plugs themselves? Touch the probe tip to the terminal on the glow plug.

Just to humor me, is the stop lever in the up position when you're trying to start the engine? And check fuse 7 while you're at it.

Sixto
87 300D