How to replace side airbag that went off - peugeot 406?

Hi, I have a 99/b 406 1.8 SV (6 airbag model) and I recently had an accident with my 406 and the passenger's side airbag (the one in the seat) went off. I want to replace it, so I bought a new airbag from peugeot and I also found one ecu that is located near the seat (not the main ecu as I understand which is located at the front) from a friend who broke a 406 to parts. The question is: what do I need to do to fix this and make the light go out? Do I need this ecu or do I just have to install the airbag? Also, does someone know the exact location in order for me to replace this ecu, if needed? Is it under the seat? I have no problem dismantling the car where needed, i just want to be certain so as not to waste time.

Thanks a lot,
Tony