I'll call bullshit to a lot of misconceptions on AM radio and reception. I still have the 3/4 inch
hole saw to bore the fender of those Fords, to install the antenna on Pre-Delivery Inspection (PDI).
After drilling the hole, running the coax and plugging it into the radio, one had to trim or match
the antenna to the radio. Length to a degree, was tuned by that adjustment.
The Shark Fin on top is for one's cellphone with a ferrite wrapped part for the Low Band (A/M).
It has to be high and in the clear as the bands cellphones are a lot higher sometimes, in the Gigahertz
rather than 1 Megahertz and lower. They are primarily line of sight (Cellphone) where the lower bands may be Worldwide!
Internal noise from various electronics in the vehicle causes the problem, not the noise floor!
Poor to no filtering and lack of development of the radio is the problem. I have a Yaesu FT-950 (ham radio)
that picks up the AM band just fine with a short piece of wire.