AM radio in my 1985 Ford F-150 works, but FM is nothing but static! Why?

Why... means you need to buy a 2020 Ford truck, that's got a thumping AM, FM sound system. Let that old clunker go. 

hold it hostage until it starts listening and  playing it.SHOW IT WHO IS BOSS

Double check the antenna- see that you still have one and that it's actually connected to the radio. If you can hear FM on a pocket radio, that tells you its broadcast is present and strong. If a pocket radio cannot pick up FM in your area, it isn't being served FM broadcast.

It's not working because it was made 35 years ago

the antenna plug has fallen out in back of the radio . 

have you thought it might be broken .. after all the radio is 35 years old .. or maybe the antenna is broken ..

Bad antenna or the radio is junk.

I heard free fm stations were no more thanks to pay radio 

 Back in 1985 , AM radio was more popular than FM radio 

Either the radio is shot. It is 35 years old or perhaps the fm antenna is not connected properly anymore. Or you might not be in range of any fm stations.