Get a 3 position 15 ampere single pole double throw with center off position switch. Interrrupt [cut/splice to switch as needed] the wire from pump to FPCU.

Fuel pump control unit 0------------0fuel pump-----------------0 Frame ground

To the left the fuel/pump is connected to FPCU
Center off = no connection to pump
To the right the pump is grounded to frame chassis and runs without FPCU.

Or just remove the fuel pump fuse or insert a purposely blown fuse.
Or disconnect the cable to FPCU.

Lots of fuses will stop car from cranking, running.

What about a time delay switch module that cuts pump off after a preset time in seconds.

It is assummed that the switch would be mounted inside rear storage to avoid lengthening wire [too much] from pump to FPCU which would add resistance and skow pump rpm down.

Would the change be noticeable if the kill switch was mounted somewhere around the drivers seat? Thats around a good 10 feet of extra wiring.

Round trip you might need 15-20 feet of extra wire [number 8 solid copper would give you ~~ 0.4 volt drop stranded #8 ~~ 0.5 volts based on 20 feet round trip.

http://www.stealth316.com/2-wire-resistance.htm

Obviously the pump draws more at WOT than idle cruise but why not design things correctly.

You could use a relay switch to alleviate the large wire just to open circuit with the switch in rear as an emergeny bypass.

Assuming ~'94 model.....If your purpose is only theft prevention and not to also have a "limp" mode to get home if the FPCU fails I would suggest just wiring your kill switch to the fuel pump relay. Only cut one of the coil wires and not the higher amperage fuel pump leads. Wiring it this way eliminates the issue of lenthy wires dropping power to the fuel pump. You are only killing the relay which is only drawing about .5 amps.

There are a lot of disabling options on the Q45. You can get pretty creative if you really want to. I've done kill switches wired through the parking lights, cig. lighter, power window switches, cruise control switches, etc. on various vehicles.

That would be a lot of trouble to go through and why have a kill switch you have to get out of the car to activate? That would be like advertising it's location. There may be a vital fuse located in the interior fuse box into which a kill switch can be spliced in series. Otherwise it's easier to get underneath the fuse box in engine compartment, splice into pump relay and run wires into cabin. Anything going all the way back to FPCU would be a long, tedious install.