When I had my Pre Plus and Pre 2, I had this vent mounted touchstone and it worked like a charm. All I did was stick the back of a Cellet brand phone holder base (after shaving flat) to the back of a touchstone and ran the power cable through the vent. It was a thing of beauty...
But now with the heavier Pre3 and its curved back, sitting still it will still hold on the touchstone and charge, but driving around will cause it to fall off. A solution was needed.
I went to Fry's Electronics and got a Scosche IUH3 Mobile Device Vent Holder
Fry's PLU# 5560920
Cost $19.99 ($10.44 if you have a friend that works there)
What I did was place the phone on the touchstone and opened up the feet on the holder and placed the phone and touchstone on the holder to where the bottom of the phone rested on the feet Then I held the touchstone in place and used a blade to cut a line in the foam right where the edge meets. Then I peeled the top part of the foam that was under the touchstone.
Then I took the blade to the edge and peeled the raised plastic outline to make it flat
Then I prepped the back of the touchstone (which I had previously removed all the foam from the last application) with the included alcohol pad and the area of the foam I had removed on the holder. Then I stuck the included 3M 2 sided tape to the back of the touchstone and trimmed the excess with the blade.
Carefully stick together and viola.
All pictures taken wiith my Pre3 save for the ones with the phone in the picture