not sure if it will work but it should ;
If you go into your fuse box and find the relay for the fans if you remove the fuse box or gain access to the terminals on the bottom of the fan relay while its still connected and get a multimeter and search through each one of the relays terminals untill the relay clicks, the terminal on which it clicks should turn on your fans too. This is the terminal which you want to add your switch to, earth the other side of your switch you should be able to have a manual overide of your fans. just make sure that the terminal is a ground not a live wire.
I hope this works, its the way i want to do mine at the weekend ive thought about it for a bit but not tested it out yet so i dont know if the terminal which clicks is live or a earth.