So I cleaned the connector at the body for the negative (ground)
cable. Did not look bad when I took it apart.
Still, took a wire brush to the bolt and cleaned the cable and body.

Then went to the strip fuse in relay. Fuse looked great.
Noticed as I was unscrewing that the screws were not real tight.

After careful inspection reinstered fuse and snugged up screws tighter
than they were.

Cannot say which condition caused the resistance.

But... I started connecting one glow plug at a time to the harness
and turned the key to on after each connection to check voltage
at the glow plug harness. After each connection I continued to get 12 volts
at the glow plug. I was waiting for it all to drop off again, thinking that
maybe one of the glow plugs was grounding/shorting.

Got to the last one and still get 12 volts when key on.

Buttoned it all up and car started just like I am used to.

Thank you everyone.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250