without knowing exactly what model Dell you're working with, I'm only going to be able to talk in general terms, but PC MB's all have a set of pins for the PC speaker, reset switch, power switch, HD light indicator, etc and a bunch of plastic connectors that slide onto these pins, you want to find the PW SW connector and splice into those two wires and then just run your pair of wires up to your external ON/OFF pushbutton that you've added to your cab.
I wouldn't even bother tearing up the chassis, just open your case and do it more from the MB end, then just run your wires out of some holes in your case, etc.