If your car still has a tape deck and you're not using it, Instructables user alanreid shows off how to build a smartphone holder right into that deck to make better use of the wasted space.
Using a cassette, a coat hanger and some tape, the smartphone holder's pretty simple, but the final product works well. You'll basically just bend the coat hanger to hold your phone, wrap it with tape, and then connect it to a spare cassette. If you don't have a tape deck, you can build a car mount for your phone in all kinds of ways. Head over to Instructables for alanreid's guide.
Reminder: in many states of Australia, home made smartphone mounts aren't legal. Check our guide to see if that applies in your state.
Cassette Deck iPhone Holder [Instructables]