I wanted to make use of the integrated phone/GPS holder port of my wifes Fiat 500C I think it is called MyPort in the Blue&Me system of Fiat. I found a model on thingiverse which fit the car, but not the existing ball joint of an existing phone holder. I made the balljoint part slightly bigger and now it fits perfectly.

Be aware that you need additional parts: cardla part of a phone holder with balljoint, 2 M4 screws and a spring, e.g. from a pencil. I addionally glued both parts together and also printed everything in ABS as it can get quite hot in a car.