They sell small FM transmitters that run off 12V,for between 10 and 30 bucks,that let you use another device like a I-pad or phone to provide the song library,just plugs in,and you receive it on your old FM stereo with no alterations,it simply picks it up like any other station,it is tuneable so you can pick a frequency that has no other stations broadcasting in your area..
They also sell fake cassettes with a audio cable and headphone plug to let other devices get used on old radio/cassette players..I have a few,they work OK..
(Yes,I know one of the above threads already mentioned these...

--BTW,the one I had did not work on both channels in every stereo I tried it in--I think the cassette head wasn't in the exact spot it needed to be in for both channels to work ? )..
I was pretty surprised to see hardly any older factory radios at the swap meet this year--usually they are a hot seller,maybe they are finally all "used up"-?-the few radios I did see were best described as "parts carcasses"..

..
..locally I know of a few salvage yards who pulled and kept a lot of radios,old school busses or tractor trailer bodies piled knee deep with them were not uncommon..some are still useable,some yards kept them stored indoors to avoid theft...
Being a pack rat I kept nearly every stereo I took out of my vehicles,even ones not worth squat like Audiovox and Spark-O-Matic ,also have a few shaft style "better" brands,but none are any prize winners--just something good enough to fill the dash hole that still worked OK...also have a milk crate full of those 70's & 80's graphic equalizer/amp things too...

..