My garage backs up against the family room in our house. During the summer, the cars help this wall get very warm. The wall is finished on the garage side so I am not sure if I want to tear out the drywall to put in more insulation. What are my options to increase the insulation on this wall?

Since you don't want to tear down the wall, your only solution may be a radiant barrier applied directly on the wall itself. This is an aluminum coated mesh board (Celotex comes to mind). Place the aluminum sheet facing the automobile so that the heat is reflected away from the wall. It is relatively inexpensive and can be installed with any type of construction adhesive.......