Where is the correct place to put a heating pad in a bearded dragons cage?

I would not advise a heating pad because when it gets too hot it can burn your bearded dragon's sensitive belly. A heat lamp is enough. There is one more lamp that simulates artificial sunlight you need too (i forget what it is called at the moment) but there is no need for a heating pad. Just, depending on your bearded dragon's size, get the correct size heating lamp. Ask your local pet store owner or bearded dragon breeder what lamp size is best for him/her.