Yep you need a good ground on the antenna. in some cases you need to get a good antenna, with 93ohm coax on it. mobileone
Mobile One AM/FM Antennas here in Aus have good antenna's as such, with any length coax, joins or extension in the coax is no go!!
I have got antennas with 11m of coax on it, works as good as a short length, ps. the longer antenna whip is best!!
It is possible that in some cases, the like of a , alternator, electric motor, solenoid.... could be faulty. when the alternator on our tractor slowly died over a month, the electric gear shift when stupid, and some other thing i cant remember
I had to put these antenna's on to overcome the GPS fuzz, and found i had to use the same antenna at the house and shed, i had no luck with any other antenna, i have tried many different antennas over the yrs