I struggled with car mount for my iPhone for a long while. I wanted something easy to use, simple, discrete and cheap. After long search I found it – magnet mount.
I have been using this mount in my car for almost a month and really did not notice any battery drain issues, quality in the reception nor GPS issue. It simply works!
1. get magnet car mount, I purchased mine on Amazon under $8, the package includes two disks – larger is the actual magnet, the smaller is a metal disk

2. mount the magnet on the car dashboard – it is self-adhesive mount and really takes few seconds

3. open iPhone and put the small metal disk on the inside of the back cover, the disk is self adhesive.I have iPhone 4s and had to use special pentalobe screw driver to open the phone, I purchased the screw driver here

4. close the iPhone, you can notice very smell bend where the magnet is

5. the magnet is very strong, it stays in the same position even with charging cable


small note: if you use full cover for your iPhone, you can place the small metal disk inside the case (no need to open the iPhone)