The paper describes a collaborative platform to support the development and the evaluation of cars interior by using a Mixed Prototyping (MP) approach. The platform consists of two different systems: the 3D Haptic Modeler (3DHM) and the Mixed Reality Seating Buck (MRSB). The 3DHM is a workbench that allows us to modify the 3D model of a car dashboard by using a haptic device, while the MRSB is a configurable structure that enables us to simulate different driving seats. The two systems allow the collaboration among designers, engineers and end users in order to get, as final result, a concept design of the product that satisfies both design constraints and final users' preferences. The platform has been evaluated by means of several testing sessions, based on two different scenarios, so as to demonstrate the benefits and the potentials of our approach.