Well, it's "both"... and I offer my engineering skills free to the forum (and especially you!) ALWAYS.
So here is the problem:
At a minimum, I need to monitor / splice into:
Hot 12 (to leave Arduino hot, so it doesn't require dead time... then program it to sleep on key off, blah blah blah )
Ign 12 (to trigger Arduino to start "start" sequence )
Switch Kill Line (to trigger kill signal)
Start button line ( to turn pump on during crank)
And prob more!
It is basically asking the basic user to solder into like 10 wires... Just not worth it...
If you think different. Like I said, free engineering time. Always and forever for you!