Found the culprit and its not the switch. There's a "High Blower Relay" on the firewall that provides battery voltage, avoiding the resistor assembly, to run the blower on high when the relay is energized and graduated voltage thru the resistor assembly for lower speeds when the resistor is not energized. By jumpering the contacts in the blower connector the blower would run on high and, alternatively on lower speeds. The suspect is convicted!!
Took the cover off the relay and tried, unsuccessfully, to repair it. Looked on the Lincoln parts suppliers web sites but didn't find the relay listed. It was after hours last night so I didn't call to check on availability and price but, rather, decided to try to use one of the 40A universal relays left over from previous projects. Blower current draw is 25A on high so 40A relay should be durable.
I wanted to remove the female blade terminals from the original connector and install them in the connector for the new relay but no dice. The terminals on the heavy wires were too beefy to go into the universal 40A relay connector. I took pictures of the process but they are too blurry to post. Don't know if its the camera or photographer, but I cannot take good close up pictures. Sorry, I know you guys like pictures.
So, Plan C was to remove the wires/terminals from the original connector, isolate each one with tape, and slip them directly on to the universal relay. A picture of the result is attached. The green wire provides a ground for the relay. The original relay grounded thru its metal body.
The universal relay was a 5-prong SPDT. I think the common use for this relay is to switch a single power source between two different loads. One load gets power when the relay is not energized; the other load gets energized when the relay is energized. In this case, however there is one load (blower) and two power sources (one from the resistor assembly for low speeds and one directly from the battery for high speed. No matter, the switch doesn't care which way current is flowing.
This is how the original wires attach to the universal relay blades (included so I have a record):
#85: A ground is required. A lead goes from #85 to the mounting screw for the relay.
#86: O/BD to the high pin out on the blower switch.
#30: O/BKS to the blower motor.
#87: BK/LT.GR.S to the battery.
#87A: DK.B/WH to the resistor assembly.
I like the universal relay solution 'cause it didn't cost me anything. IF an original relay could be found it wouldn't be cheap.
I didn't realize how dirty the firewall is. Gonna have to wash down the engine bay.