I will mainly focus on equipment that is no longer manufactured or supported by it's builder and have also included non vehicle things like portable radios, accessories, and hard to find license plates.
Prior to that I worked for Motorola service in the Chicago area and later in the Navy at N.A.S. Glenview as a mobile radio tech, and designed and installed Microwave systems for IDOT's Kennedy Expwy system. ;
I was also a member of APCO from 1967 to 1994, serving on the National Operating and Procedures committee. (APCO were the people who wrote the 10 codes and got the 911 number and decided that "car 49 from car 45" was better than saying "car 45 to car 49 ", all that stuff.)
(Don't buy those 1956 model cars with "Dial 911" on the side, the number didn't exist then)