Wipers won't shut off most of the time, even when they are in the off position. Failed switch or failed relay? If relay, where is it located and does anybody have a spare?

The most likely cause is the parking contacts inside the wiper motor are malfunctioning. They are supposed to supply power to return the wipers to the bottom of the windshield after the main switch is turned off. They could be sticking or otherwise not shutting off when the park position is reached. It is unlikely, though by no means impossible, that the switch on the dash is at fault. 72 and older 140s do not have intermittent wipers, therefore there is no relay.

I got the same advice from another board. Is the motor under the dash or under the hood?

Way up under the dash. Take out the glove compartment, that will allow you better access from the passenger side.
Good Luck
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Antique Swedish Steel 71 142E color V#102