Not sure if it helps but this is from the -20 manual...I relize it isnt about outward stuff coming in but it does describe the system other than the basic ignition parts and such:
Section VII. RADIO INTERFERENCE SUPPRESSION
2-18. General.
Radio interference suppression is the elimination or minimization of electrical disturbances that interfere with radio reception or disclose the vehicle location to sensitive electrical detectors. It is important that all vehicles, whether equipped with radios or not, be properly suppressed to prevent interference with radio reception in neighboring vehicles.
2-19. Interference Suppression Components.
a. General. The radio interference suppression system does not include any primary suppression components specifically used for this purpose. All
components of the cargo truck and ambulance that are capable of emitting radio frequency signals are equipped either with in-line (bypass) capacitors and/or are grounded to the vehicle chassis.
b. Spark Plugs. The resistor type spark plugs incorporate an extended steel base which shields the ceramic insulator and provides attachment facilities for the shielded spark plug cables.
c. Spark Plug Cables. Fully shielded spark plug cables are used in conjunction with the shielded resistor spark plugs. The metallic conductor is multiple stranded and double insulated. A spring jacket surrounds the cable insulation and provides rigidity to the cables in higher temperatures. The electromagnetic shielding is a double layer of close weave copper braiding. The shielding allows full flexibility of the spring jacket and still provides continuous shielding. The copper braiding is soldered to formed brass tubing at the ends of the cable. The center conductor is terminated at both ends by a copper alloy spring; is sealed by a rubber sleeve with a shoulder that fits inside the spark plug shield; and expands to a tight fit when the cable connector is tightened.
d. Distributor and Ignition Coil. The distributor and ignition coil are of unit construction within a continuous casted aluminum housing. The housing has provisions for connecting the spark plug cables
as described above. The housing is grounded to the cast iron engine block by the mounting bolt.
2-20. Replacement of Suppression Components.
Refer to the pertinent maintenance paragraphs containing removal and installation instructions for components.
2-21. Testing of Radio Interference Suppression Components.
Since there are no primary suppression components in the system, testing is accomplished in conjunction with electrical troubleshooting procedures.
Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.
Pro-choice, that's a LIE, babies don't choose to die!!