George Kirk basically restored a museum piece, a 20th century car. We can assume the car is long discontinued by a manufacturer long gone. So what would be unusual about putting a discontinued stereo by a long closed manufacturer? It seems totally plausible that he could have found an old used stereo system by an out-of-business company to put in his out of manufacture car.
Except it's clearly receiving an incoming phone call -- what did he put up a long discontinued transmission tower out near the barn too?
And I'm not talking about other films here (and yes, product placement and brands most certainly does date them), I'm talking about Star Trek which has never featured product placement like this. Would everyone have been perfectly happy if the tricorders in The Motion Picture had a big fat Commodore
logo on the back?
I'd also like to highlight the Beastie Boys song -- not because it's cringeworthy, but if you're going to do that, where was the more classic Planet Rock? Perfect soundtrack to Vulcan getting sucked into a singularity methinks.