cigarette lighter adapters usually work through the am/fm radio. It connects with bluetooth to your phone and transmits that through a radio frequency for in theory your car radio to pick up. The quality can suffer though.
There are some adapters that plug into the CD changer port of the stereo. You can then connect your phone into that by a variety of ways depending on what you buy. I bought something like that for my 156 stereo. It works ok.
In reality it's best to just fit a new stereo, and really a dedicated amp and new speakers but those last two are optional. I only kept my original radio as I prefer the look of it, but sometimes I'm quite tempted to put in a modern unit with a separate amp etc.