My blower (51 Xsara 1.4) came on full the other week, had no option but to take it into the local citroen dealer as I needed to get it fixed quickly. As well as the blower on full the aircon did not engage, the green light came on when the button was pressed but no cold air. You are correct, the blower has a built in controller (on the multiplex bus I believe) and can only be brought with the complete blower unit. The blower costs around £90 but with the diagnostics, fitting and reseting the computer came to just under £200. 'chard