If you burn your own CDs you should be extra mindful when you apply a CD label to the CD. If you aren't careful and you don't get all the "bubbles" out by smoothing the label firmly across the CD surface, the label can warp in high heat and start to come off. If this happens in the magazine, and, ultimately, in the player, you could end up with a CD that gets jammed inside the reader. Again, I've seen this issue only when the label is not applied correctly. My burned CDs, for example, have never bubbled or loosened--and I live in temperatures that often exceed 100 F. If you aren't sure about your labels, keep a couple of labeled CDs in the trunck--outside of the magazine--and inspect them after a few days of intense heat to see if there is any sign that the label is loosening.
In general, there are no issues keeping the CDs in the magazine in the trunk or even in the cabin (for dash-mounted systems).
By the way, CD players were available as an option in most cars starting in
1988.