Pebee;

First step in quieting down a car for a radio installation is determining by which mechanism it is entering the radio...by way of the antenna (in which case it is likely ignition noise), or power line (in which case it is more likely Generator brush noise)...each interference has its own specific remedy...

...and Chris has also touched upon it...generally when radios were installed (by the dealer) it was accompanied by a "radio installation kit" which quieted both the ignition and power buss, so if you are going to retrofit a radio now (where the car had none previously), you will also need to do make the modifications necessary to allow quiet AM listening when the engine is running (FM is inherently somewhat more immune to noise)...

Recommended reading: https://www.sw-em.com/Radio_Notes.htm#Radio_Interference

Good Hunting!