All flowing electricity (current) produces an electromagnetic field. That's how devices like a clamp-on ammeter can read current without actually touching the wire.
A defroster uses quite a lot of current, often, 10-15 amps flowing through un-shielded heating elements in close proximity to the antenna (mounted in the glass, on the roof above the window, or on the fender/trunk lid near the glass).
To put that in perspective, most houses are wired for 10amps per room. That defroster draws more current than all of the electronics in any given room in your house.
All that current in such a tiny space produces a pretty large electromagnetic field. Radio waves received by the antenna are just electromagnetic waves at a certain frequency, say, 106.1mhz for my favorite local station.
With a powerful field so near by, interference is caused, and reception quality is effected.
Anything drawing that much current, whether it's lighting, an amplifier, or a power inverter, will produce the same effect.