The connections are the following (mcp2515 - arduino):
-INT - 2
- SCK - 13
- SI - 11
- SO - 12
- CS - 10
- VCC - LM7805 3rd leg
- GND - LM7805 2nd leg
(12Vinput: positive - LM7805 1st leg, negative - LM7805 2nd leg).
Why do you think that the car's battery, or a 12V adapter is not reliable?