How to wire car battery to arduino and relay module

Hello all, Please help me with this. I´m a newbie. I have an arduino wired to my car 12v, thought a step down buck converter (5V) to power my arduino. I what to use a relay to switch on and off auxiliary led lights. How can i wire the arduino to the relay module? I have the arduino programmed with a canbus shield to detect high beam lights. Then a signal will turn on the aux lights. I´m not sure about the ground wiring, because i´m using a common power source (car battery). Please help.

If you are using a relay ( mechanical or solid state), you should not need to worry about grounds, as they will be isolated. Do you have a relay board? Which relay board?

FYI: auto power has lots of noise. Use extra capacitors on the power bus, and if you are using any analog inputs, extra caps there.

Thank you Jack for your reply! I don´t have yet a relay module. I don´t know what to use. It has to be an external module. Just need one relay. Simple on an off operation (i think an SPST??).

I´m thinking in getting the relay inside a small waterproof box halfway between the arduino and the led lights. The aux led lights draw 40W 3.2A total.

The arduino is gets is power not directly from the battery, but using the canbus 12v. Then the relay will use de 12v from the battery and wires from arduino signal.

I also have a spare MOSFET Power module from Sparkfun, that i first thought it could replace the relay... but from i have read most people use relays. Am i right?

Regards, Luis.

"I also have a spare MOSFET Power module from Sparkfun, that i first thought it could replace the relay... but from i have read most people use relays. Am i right? " I think a relay is simpler. If you are not really familiar with the MOSFET, I would suggest a relay.