Webasto have a remote radio control unit, I use a T70 on mine that has a good range. I have heard of installations where, depending on the alarm unit installed, an aux output can be used for the same purpose. You can normally find the remotes on ebay although there seem to be more in Europe, I got my kit from Germany as the cost for a new one was the same as 2nd hand ones over here. Alternatively you can fit a timer unit which is cheaper & if you normally leave home at a set time can achieve the same result. Cheers.
Neil