renault megane senic t plate. batery light and stop light staying on ?

Renault megane senic 1999 t plate. I started around a week ago, at first the batery light started flashing then the stop light came on and within 5 mins the stayed on the car broke down. there was no power at all i then called the RAC out they put the volt meter on the car he said the alernator was broke as there was no power coming from it. I then brought a new alernator, had it fitted on the car and the stop light and batery light were still on. I put the batery on charge over night then put it on the car and the lights were still on. I have just put the car on a diognostic tester and its saying that the alternator is not putting ang charge in to the batery well a few volts only. iv checked for any loose earths and i have not reciently changed anything on the car i.e done anything to make a wire come lose? my mate is a mechanic and this has confused him as i said it was a brand new alternator.