The material you see is a potting compound. It is not melting, but is slowly breaking down and oozing out. Potting compound has been used for decades in electronics to seal for environmental protection against moisture, debris, etc. and movement of internal components from vibration, mechanical shock, etc. The compounds are typically fast curing two part or single component chemicals that form a solid or semi-solid protective barrier. Over time the compound can break down and "run" or otherwise deteriorate. Heat does not necessarily cause it, but can aggravate the breakdown.
Chrysler used semi-solid potting compounds in the electronic control units (ECUs), electronic voltage regulators, air conditioning high speed cutout controllers, EGR timers, and other similar electronic modules. In some ECUs and voltage regulators, sand was added to the compound as an additional stiffener for mechanical shock and vibration resistance.
1970s assemblies are more susceptible than later production parts. Later production was changed to a more rubber like compound that breaks down less. In general, the parts will continue to function with the compound leaking out unless affected by moisture or other factors. Aftermarket components can also suffer from similar breakdown depending on the nature of their compound.
When the early compound used by Chrysler runs it forms a sticky mess. It can be dissolved with lacquer thinner, but it's tricky to remove from painted surfaces without risking the paint. I've successfully removed it with lacquer thinner from vehicles as well as ECUs and voltage regulators, including the internal areas. Removal from the internal areas does risk removing electronic component markings and weakening the components, especially the electrolytic capacitors. I have replaced the removed potting compound with two part (A-B) silicone or urethane compounds.
