Depends on the original configuration. If you have an amplifier then the B connector never existed. The amplifier preamp outputs are on the C (yellow) connector.
Look here:
https://www.pelicanparts.com/techarticles/Boxster_Tech/88-ELEC-Radio_Install/88-ELEC-Radio_Install.htm-
If you never had an amplifier then the speakers were wired directly to the head unit and you can recreate the B connector as the plug socket is the same as the A connector.