Thanks for keeping me on track, Art.

I like the test lamp idea; I've used one for years but the low-watt bulbs I used wouldn't be much better than a fuse, so now I have a freshly-made one that looks remarkably like yours. Finally found a use for a dual-filament halogen lamp with one filament burned out.

Armed with the new test lamp I proceeded to work from the compressor back to the dash, unplugging each component in turn; no change, and the AC works fine. Then I realized that yesterday's trouble-shooting may have unknowingly removed the problem. Recall I've bypassed the low pressure switch by removing the connector and jumpering across the connector terminals. When I was poking around yesterday, I unwrapped the connector, checked the jumper, then re-wrapped it all and re-secured it with a tie wrap. I suspect the problem may have been with that arrangement. Although nothing looked arced or corroded, the spring clip on the connector was exposed and it may have contacted the jumper and allowed a short to ground.

So all's well for now, but if the problem recurs, I'm better armed and educated to diagnose. Thanks for the help!