I'm not familiar with M-37 but all other Mopars I know of use a voltage regulator, usually right on the instrument panel, to reduce supply voltage to 5VDC. M's are 24V, so it's a 24VDC-5VDC regulator or converter.
These old one are crude, using leaf type contacts similar to a reed switch. The contacts can get corroded or the whole thing can get grunged up cuasing erratic operation.
There's a conversion process for 12VDC systems for changing to solid state circuitry. It consists of a +5VDC Voltage Regulator, and Elecetrolytic Capacitor & a Heat Sink. Radio Shack P/N 276-1770a is a voltage regulator rated for 35VDC input, converting to 5VDC output, which works fine for 12VDC input supply, should work for 24VDC. Radio Shack 272-1014 is a 22mfd electrolytic capacitor, and the heat sink is anything that will fit, the larger the better.
If you have the little rectangular 5VDC regulator on instrument panel then that's the usual problem.
I posted at Dodge Link for Richard Ehrenberg tech info on the problem.