And now, coming over from the hosting thread, we get into radio, which is the other large part of my career, lol. Though my radio expertise is in the form of 2 way radio (think police radio type things), a lot of it transfers to AM/FM reception as well.
There's a lot that goes in to a proper antenna design, though a receive only antenna is a hell of a lot easier to deal with and is much more tolerant of bad design or installation.
In short, just about anything metallic will work as a radio antenna. This is why shoving a paperclip or screwdriver in the antenna jack will pick up radio stations. To work well, especially in areas with weak signals, you need an antenna specifically designed for whatever frequency you're trying to receive, and bigger isn't always better. This is why trying to use the old 102" CB antenna often works poorly compared to the whip that comes on most cars.
However, metal is also very good at blocking radio signals (Google Faraday cage). All things being equal, an antenna outside a metal body car will work better than one inside. Something under a steel dash can be just about useless.
No, it doesn't have to be an ugly whip on the fender. Think 70s/80s GMs with the wire embedded in the windshield. Yes, using the car body as the antenna works surprisingly well, but you may also end up picking up a lot of electrical noise as well. I did this on my Scout in the 90s when I was a dipshit teenager that didn't know any better. Now I'm just an experienced dipshit adult, lol.