I recall being told that CDs were bad to keep in cars because of the heat.

I always kept my CD wallet (a small one with 10 discs that I’d switch out every now and then) under the seat and never had a problem. Before I had that, I had a CD case that kept maybe 20 discs in their jewel cases that I kept behind the driver’s seat, and no issues there either (though that was MUCH harder to swap discs while on the road). I also had a visor holder that I later used in the motorhome with MP3 CDs (now I could take my whole collection!).

In the early 90s, I paid $300 for a very basic in dash Sony CD player with output for only two speakers. Somewhat early adopter tax.