Well, I think the best thing to do would be to buy an open circuit switch. If you really want to use the kill tether from your blaster you could wire in a Bosch relay that will send power to through one circiut when no voltage is applied and to another when the relay is coil is activated. You can wire the kill tether into either the 12v+ feed, or the ground feed, which will go to termilal 85 or 86, it makes no difference which terminal is positive or negative. Wire the ignition feed into terminal 30 and out of terminal 87a. When the tether is connected, the relay circiut will remain closed keeping power flowing from terminal 30 to terminal 87a and the ignition will stay on. When the tether is pulled, it will close the circuit and energize the relay coil, switching the current flow to terminal 87, which is not used and therefore opening the ingnition circuit, killing the engine. If you need further clarification, google Bosch relay, or SPDT relay.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.