Do you need to replace an entire sound system?
Or are you looking for an adapter to play a certain device through an existing sound system?
The system must be compatible with the number of speakers, and the wiring to the car chassis.
To be most useful, your system should be compatible with modern devices and file formats. MP3, etc. Consider having more than one cigarette lighter outlet.
You should avoid doing anything to the car that could make it difficult to sell later on.
There are any number of adapters for all kinds of devices. See what is offered in automotive parts stores.
You can play a CD through a cassette player. (Dummy cassette housing contains an inductive element positioned near the tape heads).
You can play an Ipod through the car's sound system. (Possibly through the radio.) Etc.