fbdev driver on Wheezy

[Andrew Wood <andrew@perpetualmotion.co.uk>]

Im trying to get Wheezy with lightdm and Xfce to work on an old kiosk 
machine with a Silicon Motion SM712 chip. Currently X just turns the 
screen off as soon as it starts and from the Googling Ive done it seems 
this is a common problem with both the silicon motion driver and the 
generic vesa driver and the solution appears to be using the fbdev 
driver instead?

How can I force X to use this driver under Wheezy, there doesnt even 
seem to be an xorg.conf file anywhere?

Thanks