Hi, not as sophisticated as commercial variants but a very good low cost alternative....get a large sheet of very light polythene, big enough to drape all over car and maybe 500 mm excess all around, decorators polythene will work. Drape this over the car and place a 45 or 60 watt tube heater underneath the car. Very low running costs, polythene "inflates" with warmed air to form a chamber and air bleeds out of edges to maintain a flow so naturally dehumidifies.