air con totaly dead on my 2004 scenic 19 diesel?

had heater fan repaired a while ago ...air con worked fine prior to that,when i got it back and air con switch dont light when pushed checked every fuse in the car all fine borrowed a curcuit tester to see if switch is faulty. not done that yet,im thinking that when heater fan was repaired then had all the dash put back they,ve left a plug undone broken a part or something, i removed heater switches to make sure the plug was connected there, when i phoned them back and told them it wont work they didnt want to know i bought the car from them and the fan repair was done on warrantee ,,,but by that time it just about ran out.. everything else in the car works fine ...all heater controls and fan speeds good. just no engagement of air con