For example if you were fitting a new radio into a 2007 bmw mini which has a fakra plug on the car but the new radio has a din socket you would need the adaptor PC5-100 made by autoleads, if you search this part number on ebay you will find many sellers with a picture so you can compare with your car.
Alternatively if you were fitting a new radio into a 1997 ford puma you would need PC5-27 autoleads part.
Some cars maybe vauxhall, seat VW as well as needing an aerial adaptor will need an aerial amplifier such as PC5-137 or PC5-52 or PC5-91but there are too many of these variants for me to list in this guide. If your car needs an aerial amplifier and you dont fit one you wont do any damage but you will not get as good radio reception as you should be getting.
A PC5-90 is the amplified version of a PC5-27.
A PC5-137 is the amplified version of a PC5-100
If you search ebay for these part numbers i have shown above and look at the pictures my guide will make much more sense, if you found my guide helpfull in any way please vote for it.