Useful information and websites
Here are some very useful websites. A massive thanks to them for existing. We use these websites ourselves every single day to find information about some radios. I do know that they are a bit messy and not in English, but hey - they are here!
Here are catalogues for aftermarket radio brands:
https://antique-autoradio-madness.org/page-liens/autoradios-eur.htm
And here are catalogues for original radios by car brands
https://antique-autoradio-madness.org/1autos/liens_autosdef.htm
This one is also very helpful for finding models using a search engine
Pinouts
The best two websites for car radio Pinouts by makers, thanks to them for keeping all this information.
https://connector.pinoutguide.com/
and
This is the most common question we receive. Even though we recommend using professionals for connecting all the wires, people tend to do it on their own.
So, here's an explanation of the standard ISO connector.