Switching 12V vehicle headlights with Arduino / Mosfet combination
I would say yes to your last thought.

Just keep those two separated from the whole circuit and hook your arduino and mosfet there. Keep the connections from the rest of the circuitry as little as possible.
For example you can take the output from this &quot;LIGHNING SWITCH&quot; and use it as an input to your arduino. When you press the switch, the arduino will sense it, the lights will turn on.

Something like this will do the trick:



simulate this circuit &ndash; Schematic created using CircuitLab
EDIT
Regarding the N-mos, P-mos, this is a picture of my noted regarding those:

N-mosfets have lower internal resistance (Rin) (look in mouser.com for the  mosfets, play with the N/P- chanel filters and check the Rin differences between the N channel and P-channel (Ignore the NPN, PNP etc options)
N-channel mosfets are easier to use when put on the low side (since you control Vgs, and you have a steady GND at source, you can play with the V at the gate. (This is why i have an &quot;L&quot; box on my notes, its  where i put the &quot;Load&quot;).
Also, enhancement type means it turns on when the Vgs is (lets say for logic mosfets) 5[V] or more. Depletion type means the fet turns on when Vgs is 0[V] or less.
Having said that, if you check on mouser, P-channels have easily 10 ohms of Rin (huge huge!) N-ch can easily have 0.009Ohms of internal R.
Also, if you check on Mouser the &quot;Depletion type&quot;, you will notice that there are no P-channel mosfets with Depletion type. That is, becouse they are expensive to manufacture, they have higher Rin, no one wants them.
This leaves you with enhancement type P-channel mosfet. So, If you want to power your bulb with 12V, you put it on the low side, how will you turn it on? you need Vgs = 5V, you will need another supply of 12+5=17V, like this:



simulate this circuit
That is why, it ususally is a bad idea to use a P-ch. Only when you really need a device (load) to be connected to the GND directly, you would put a P-ch there (like if you wanted to power a computer, motherboard, MCU etc)