You sure it's the BSI 'cos 406s also have a whole mass of connectors under the driver's side carpet that give some interesting faults when they get wet.
Yes they can be repaired, however your car is off the road for the duration. I believe the BSI controls , imobilliser, aircon, and a multitude of other functions. Put BSI in Google.
To see if can be reapired try WWW.BBA-REMAN.COM
If the unit is mounted under the dash, then either the air box drain tubes are blocked, or the windscreen is leaking.