You may want to try this thing that I bought off of eBay. You get a 1.5" dia flat magnetic piece to put inside the back of your phone case. Then the main part slides into/onto your A/C vent louvers on the dash. Then your phone just stays on it magnetically. Works well and it is pretty cheap....
Stalion®Stand Universal Cell Phone GPS Air Vent Magnetic Car Mount Cradle Holder | eBay