The primary relay is held on by a ground loop thru the relay itself keeping it energized once the magnet is removed from the reed. The kill switch is inline with that ground, depressing it (momentary open) deenergizes the primary relay, & subsequently the secondary relay.

The secondary relay goes in the positive side of the ignition coil wire. Basicly it comes off the ignition fuse, into the relay, & on to the coil.
Now this same setup could be used to kill an electric fuel pump, or even do both if you wanted to run two secondary relays in paralel, one for each item.

The 12V automotive relays I have used are rated for 15-30A, they are used for items like fog lamps & driving lights, so running a coil or fuel pump is no problem & the wide contact area can handle a power spike w/o fusing.
They are very resilient to bumps/shock/etc. But yes, loss of power to the primary solenoid would kill it & of course the engine.
I've run a few different varieties of relays in my baja, & never had one fail due to bumps, so I would feel comfortable recommending this setup for cars that wont see that kind of deliberate abuse.

The 12V flasher LEDs dont need an additional resistor, they are built into the unit, all they require is power.


If someone wants it bad enough it's history, period. I just wanna make it hard for the amatuer pukes most of us have to deal with.
You can hotwire most anything with a couple lengths or wire with clips on em. How far is it from the + pole on your starter & the + side of your coil & to your ign. solenoid? Two pieces of wire & it's gone!
Enclosing the connectors & adding relays in the circuts just makes it that much harder for them to get away with your baby. The more time it takes em, the less likely they are to mess w. it.