I've modified the bluetooth remote control of the smartphone to be smaller. I did that with :

- an Adafruit Pro Trinket board

- a Xbee socket for the RN42 BT HID module

I intend to fit this module wrapped in shrink tube inside a compartment of the car. It is near the fuse box and I can open it by just removing a bezel (no screws!) : it is pretty mandatory for the future updates of the firmware.