Well, I worked with dangerous stuff myself. Safety is number one thing. Always.
Because of that and also because of the fact, that we are discusing kind of "general" approach (we don't know for sure what relay, diode, resistor, battery etc. someone will use), I suggest to put battery, or any kind of power source on a control side.
Because wire can short, LED can fail, relay can activate from mechanical shock, you name it. So it is best to remove all these posibilities all together by putting battery on a control side. You can even physicaly disconnect ignition wire from control device, setup everything, go away, connect wire, push your buttons. All other details and problems can be discussed and solved. Should not be very hard to make it work.