"Do I need to be concerned with what gauge wire..."
Short answer is yes. Not to make it work, but to ensure a wiring fault later in life will take out the fuse and not start a fire.

Your ohmmeter readings make sense.

Before turning in last night, I thought I'd ask you about how you planned to use these, i.e. an existing light in the instrument cluster, referenced to ground or battery. Dave knows the 940, and gave you the right suggestions I think, but then I see you are providing your own lamp and have all the options.

Given these options, I'd locate the relay as close as practical to the fused ignition switched battery source, and wire terminals 30 and 86 together to it with a short piece of 20AWG automotive wire. 87A then to your lamp and the other side of the lamp to ground. Then, the long wire to the float switch from 85 can get pinched anywhere on its path to the reservoir and merely cause the warning circuit to malfunction rather than have to blow the fuse. The float switch interrupts the circuit on the grounded side. I think that's what Dave was thinking too.

I bought a few float switches years ago. Not sure if I could put my hands on them quickly to check out whether they float open or closed. Your thread gave me renewed interest in the idea, remembering the one and only "catastrophic loss of coolant" incident I experienced with a 240 over 20 years ago. When that happened, I had a turbo pressure gauge tapped into the overflow plumbing in the reservoir.



Not as much pressure in the 240's reservoir as in the 7/9 cars.



But when I saw the wisps of vapor, a glance at the gauge told me it was time to get to the right lane and off the interstate. I'd rather do it the way you're doing it.

--
Art Benstein near Baltimore

Sign on on the side of the electrician’s van – “Let Us Get Rid of Your Shorts”.