Can someone tell me what this plug is.

It runs from the distributor to the bottom end of the engine under the exhaust manifold?

 

See page 26 of 734 in the electrical guide for the D1 in RAVE. Ignition amp module uses those color wires, and there is a noise capacitor that goes between the Red wire and ground (frame). The schematic shows the colors as R (red) and U (blue).

If the noise capacitor fails, it will either be open (and more noise on radio), or shorted to ground. Capacitors are basically two sheets of metal foil separated by dielectric jelly, wound into a cylinder. They frequently fail "shorted".