Changing the bsi unit in 206 gti (1999)?

Hiya, my car wont function at all, constant relay sounds when battery is connected, lights on the dash flashing, and a warm smell from dashboard, we were told by RAC that its the bsi unit, we brought another one from pug shop, we were told it was a case of plug it in and change the key and away you go, this isnt our case, we changed everything and when battery was connected we could hear relay sounds and clicking, key didnt work either, it was worse than the one we took off.. so again we had to disconnect the battery, we dont know what else to try, did we fit it properly? or is there a certain way of doing it, please help