From what I've heard they're expensive .... very expensive. They aren't called "The Main Stealers" for nothing.
However, all is not lost. A member here, David Pye has designed and built a hard wired add-on switching system to do the job.
You can find out more about it at: http://www.landyzone.co.uk/lz/f8/sls-suspension-controller-kits-available-215142.html