A controversial bill that would require all new cars to be fitted with AM radios looks set to become a law in the near future. Yesterday, Senator Edward Markey (D-Mass) revealed that the “AM Radio for Every Vehicle Act” now has the support of 60 US Senators, as well as 246 co-sponsors in the House of Representatives, making its passage an almost sure thing. Should that happen, the National Highway Traffic Safety Administration would be required to ensure that all new cars sold in the US had AM radios at no extra cost.
There likely was no “shielding” in a truck of that era, just simply the truck was made of metal as was the chassis of the radio, bolt 'em together and you’ve got a reasonable ground.
But, I do know from experience that there are items on a pickup truck that can produce radio interference especially when worn. A worn distributor is a spark gap transmitter, as I learned when I installed a mobile radio in my S10. The audio on my radio got a lot better after a good service of the ignition system.