Citroen Xsara Picasso battery light won't go out

Hi, I need some advice on a charging fault on my picasso 2001. I had to jump start the car as the battery was dead.It started ok but the battery light on the dash stays on and I get a message on the instrument panel saying 'Battery Charging Fault'. I checked the voltage at the battery when started and it is only 11.5volts, so I checked the voltage at the alternator and it is also 11.5v and not increasing with higher revs. Can anyone tell me where the other two wires on the small black plug of the alternator are connected to or should the alternator charge regardless of these two wires. I don't want to replace the alternator if it is not the problem as they are not cheap. Any help would be appreciated. Thanks