Yes, you need to connect Arduino ground to the car ground (i.e. negative side of the 12v battery) as well. If you're running your arduino from the 12v car supply, you must be doing this already.
Don't forget the diode across the relay coil.
Yes, you need to connect Arduino ground to the car ground (i.e. negative side of the 12v battery) as well. If you're running your arduino from the 12v car supply, you must be doing this already.
Don't forget the diode across the relay coil.