Actually, I do have an idea
Over on the SquareD build thread, you may have noticed the ARC switch panel bank.
Go ahead, take a moment and check it out.
OK, now that you've see it. the actual switch panel simply Velcro's to a flat surface and has one small 10 pin connector about the size of a USB cable.
So, you wire the ignition, the starter, the fuel pump and anything else you wanted to that relay box which you could mount anywhere, as in out of sight. Nothing is labeled on it so no one could hope to figure it out. When you leave the vehicle, simply pull the whole switch panel off the dash and put it in your pocket. Nothing that that thing controls will then work. Voila' you're theft proof!
Bam! How you like them apples!