“Evergreen” is radio jargon for radio content that has NO embedded time references or current event items in the news that pin down when the show was originally recorded (or has them edited out after the fact). Car Talk – also known as The Click and Clack Show that airs on NPR is already most of the way there – and going further in that direction.
Nobody listening on the radio is hearing the Car Show “Live”, which is why the show is as early as 10 AM (ET) and as late as 4 PM. They will mention a web site to call to get your question on the air. If you were to actually call that number, you won’t get “on the air”, you’ll go through a process of finding out what your question is, and then when the show is being produced for the week, the show producer will call you – your question is done with the hosts – and if it didn’t go well, they have the option of just throwing the segment in the trash and and trying another call and/or editing down the conversation. It’s a way to create a highly polished show but still appearing to have some aspect of “being live”. It also gives them time to research your question ahead of time, making them sound really smart about obscure details of your problem, when what they really did is read the mechanic’s manual and made notes before the call was taken.
The Click and Clack brothers are way beyond normal retirement age and want to find a way out the door. This fall, Click and Clack will be running material that is not current – old calls, maybe material that never got on the air originally, and a little bit of new original content. A call like “Hi Guys, my mechanic says my 1987 Saab should have a new timing belt – what do you think?” – the answer and advice won’t be a lot different from 1998 and 2015 – and if you listen to the show, you already know the show has very little to do about fixing cars.