To help put descriptions in a visual sense, these images help illustrate how the stock antenna systems help shield out unwanted RFI (radio frequency interference).
Items such as ignition components emit RF that is unwanted on the radio. Shielding and component location is the key to preventing the "interference song" on the radio dial. A typical automobile is a shield in itself. The steel body panels act like a guard against RFI. The Corvettes utilizing fiberglass body panels have to use some special adaptations. Why do you think the early 'Vettes had that funky-looking cover over the distributor and ignition wires? The answer is to keep the unwanted intereference from "getting" out and making its way into the antenna system.
This basically shows how the radio antenna works. Inside the coax cable there are actually (2) conductors. The smaller inner one (shown in red) is what actually links the antenna's mast (or '70-'72 windshield antenna) to the radio's tuning circuitry. The outer "shield" that is wrapped around (but isolated from) the inner conductor keeps out unwanted interference. The shielding system is shown in black. The car's sheetmetal, and even the radio's outer case link both ends of the coax's shield to ground. When connected together, this looks like one massive shield to unwanted RF (shown as blue arrows).
So what happens when the shielding system is not in "unity" ? Well, that's when problems start...
The above picture depicts a common occurance. The antenna's grounding saddle does not have good metal-to-metal ground contact with the inside of the fender. This may be due to poor installation, corrosion, or a recently restored car that has "too much paint" on the inside of the fender not allowing a good ground contact. This basically looks like an open "flood gate" to unwanted RFI. It penetrates the weak link and now you hear "the spark plug song" on your radio because the interference travels down the inner conductor right into the radio.
There are other variations to this problem as well. If you have a damaged antenna coax cable leading to the radio, this will also be susceptible to unwanted interference. Fiberglass body panels do not have the same shielding quality that steel has. Mounting aftermarket ignition modules close to the windshield on the firewall just "invites" RFI onto antenna systems as it is in very close range to antenna connectors, regardless of how well they are shielded.
***Here's a tip to see if your "shield" has a break in it. If your radio has annoying crackling, popping, etc. when the engine is running, tune the dial to the noisiest spot. Now unplug the antenna lead from the back of the radio. If the noise goes away, or is significantly decreased, you have a shielding problem. (Most likely one end of the coax antenna lead is not grounded properly OR you have aftermarket accessories routed too close to factory antenna connections). Check your grounds first! It's just like when your headlights and turnsignals don't work, check that metal-to-metal ground contact. The same goes for antenna connectors--only instead of providing a current path, the grounds provide the much needed shielding system against stray RF.
