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These vehicles in general (that generation explorer/mountaineer) are actually know to the amateur radio community as being RF noisy vehicles in their own right.... they have noise from DC to about 140MHz that is very noticeable.
As aquanaut20 mentioned, check the radio grounds (the AM antenna may be built into the radio or it might be just a connection to the car chassis - if the later, then you need to find where as these are notoriuos for having issues).
Good luck... I gave up on using the AM radio in my truck on anything other than the local stations.
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
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