Is this for an AT or ATX power supply? If it's ATX, then go to radio shack and buy a normally open momentary push switch. This will have two connector on the back - then some thin cable from these connectors to the ATX power switch header on your motherboard and voila - a computer power switch on your dash! The same applies to the HDD and power LEDs. I bought a switch and two leds from radio shack and ran these to the headers on the motherboard so now I have the power switch and HDD/power LEDs right on the dash.
If you have an AT power supply, things get a little tricky since the power actualy runs through the switch. If this is the case, you could always use a relay mounted in the computer then run thin wire from the relay to a switch on the dash. This would be much easier and safer than running the power to a switch in the dash then back to the trunk.