what ohm speaker were the originals, and what happens to the radio if one just sticks in "any old speaker" that fits, of a different ohm impedance ? [i think that's the correct term to use]
In the 50/60/70s most automotive OE speakers were 8 Ohm...
In the 80/90s, as the OE head units increased in power by adding more channels and higher power (transistor-based) output stages, the speaker impedance spec dropped to 4 Ohms..
Today
most replacement automotive speakers are 4 Ohms, some like the Infinity reference are 2.8 Ohms.
My experience for most installs in our vintage Mopars, the biggest risk is putting a 4 Ohm speaker on an older OE head unit that was designed for 8 Ohms.
Note that a 4 Ohm speaker requires 2X the current of an 8 Ohm speaker, and this additional current requirement frequently will burn out the amplifier output stage in an older Mopar vintage head unit.
Final note, later aftermarket head units have higher current capability and also typically will have more built-in protection circuits for shorts and/or low ohm speaker loads.
Just my $0.02...
