Manufacturer's original equipment or some third party
installation? It simply takes an antenna plug not exactly
sized to the radio socket. That intermittent connection then
creates radio static. There are a long list of other reasons
including loose ground to chassis. There is very good reason
why grounds are not just put anywhere. Does the antenna coax
shield somewhere make contact with the chassis ground? That
too would create static problems.

Your car radio should receive major (50,000 watt) AM radio
stations even 100 miles away. If not, your radio may be a
discount special, or you have loose connectors, or other
possibilities are just too numerous to mention here.

Remember wire is just another antenna. Even where a radio
is grounded in relation to everything else can create
interference. However number one on your list would be an
intermittent antenna wire maybe because the antenna lead plug
does not quite match the antenna lead socket connector.
Welcome to an art.

wrote:
My AM car radio has tons of static when the engine is running. The
thing is useless when the car is on or even if the key is turned to the
on position right before you start the car. It's fine if you turn the
key to the aux position.

Another clue...

When I turn an electrical motor in the car on and off, it sometimes
reduces the static level...but never eliminates it. And then when I've
fiddled with something and reduced the static, I'll hit the brakes, and
the brake light will trigger the static onslaught again.

I have taken the radio out, and checked the antenna connection as well
as used jumper cables to ground the grounding bolt. No significant
improvement.

Thoughts?