[John Strenk]

If you have a test light or meter probes with a sharp point, you could push the probe directly through the insulation to see if there is power just a few inches away.

Cover up the puncture with tape to keep ant moisture entering at the puncture point.

 

[JEEPRANCH]

I punctured the insulation near the end of the yellow voltmeter wire-no joy.

There is, however, continuity from the big yellow wire to the yellow wire that is the unused 4cyl tachometer connector (yellow circle). Rather than unwrap the whole underdash harness, I'm thinking I'll splice into that with a new lead to the voltmeter. Do you see any downside to that?

 

[John Strenk]

I would first check to see if anything else is not working besides the voltmeter. Like turn signals or radio.

If they check out then any switched 12 volt power source will work.