The problem is, since idol is made up of a wax material, I can't use severe adhesive material. So I used wax itself to stick it to dash board. But it melts down when cars is exposed to sun constantly. So can anyone suggest me a method to fix idol sternly to dashboard? Please note strong adhesives can harm idol too.
Rather than fixing your effigy to the car dashboard directly, you can securely affix a small tray, cup, or other attractive protective container that it can sit in safely and be displayed prominently.
A good container will be a snug-enough fit that doesn't need any adhesive to hold the effigy itself.
If you cannot find a perfect fit, you can use plasticine to hold the idol on its platform for proud display.
Children use plasticine (modeling clay) to make shapes to play with. You can get in most any toy store. Use small amounts of it to discreetly hold the effigy to the platform securely for as long as you want. Plasticine is safe for children, comes in many different colours, does no damage to anything it touches, doesn't melt in the heat, and doesn't change (harden or soften) over the time you will own your car.
You may discover that using the plasticine alone to hold the idol, you do not need anything else, in fact.
Since your deity exhibits such a nice base plate, you can make use of it to fixate it.
You need something which get's a good grip on the baseplate and which can be fixed on the dashboard itself.
You can mold some sugru around the baseplate of the deity and stick it to a piece of sheet metal. The sheet metal again can be fixed to the dashboard by any adhesive you like to use.
The sugru is so soft I assume it can be cut away easily to free your deity's foot if you want to remove it later. If your deity is really made from wax the sugru shouldn't stick to it. If you want to make sure, you can apply a drop of oil with a cloth to the baseplate of the deity before molding arond the sugru.
For a start, if your car sits in direct sun for an hour or more with the windows closed, it's likely the interior will get hot enough to melt the idol, never mind the wax used to adhere it to the dashboard. Further, there are few if any adhesives other than wax that will stick to wax. The only one I know that's likely to work is RTV silicone, the kind that smells like vinegar while it cures.
Assuming there isn't some significance to the material of the idol, you should probably first get a more durable idol, made of a hard plastic like polystyrene or acrylic. Either of these materials can be glued with cyanoacrylate ("super glue") or polyurethane cement ("Gorilla Glue"), assuming your dash material is compatible with those adhesives (it most likely is). These plastics are still subject to warping in the temperatures of a car interior in the sun, but will stand up much better than wax or the waxy feeling polyethylene.