Firstly, I can tell you that silica gel desiccant crystals DO work, but you need a reasonable amount to dry out damp cars.
However, as you know, there must be a source of dampness..a common one being water finding a way to the interior, and (on some cars)water sitting in door seals after rain - it simply increases the moisture level inside (VW group cars are culprits here).
So, you need to find what's putting the moisture level up, put it right, then silica gel can help keep it dry.