Sounds like you are using one of the old AT power supplies for your board. The ATX power supplies only require a momentary swich wired to the power posts on the m/b. I have done this with my system and it works well. A push on/off switch turns on the inverter, a momentary switch fires up the pc, and another momentary switch turns on the monitor. It might be alot easier to swap to a newer power supply so you don't have to run 4 wires from the pc up into the dash.
Just my 2 cents worth.