Macchina connections and pinouts are as follows:
The header pinout is the same as the Arduino Mega, with a few minor differences, including how the SPI lines are connected to the microcontroller. Read more about the SPI connections here.
Both the Mega and the Macchina use an Atmega2560 as the microcontroller. Here is the datasheet:
Now that you have read all 435 pages of this datasheet, we can proceed.
3-Pin connector for Power and GND. Power can be 9-16 volts unregulated nasty power from your car.
6-pin Automotive interface connection with the following pins: