An in-line power conditioner and a 500,000uF filtering capacitor.

A neon lamp is attached to the top of the case to light up the interior for troubleshooting. To the right of the image you can see the magnetic reed switch to turn it on. It is connected directly to the 12v source so it can provide light with the truck and the computer off. When you open the lid, it turns on.

It is wired like this:

12 volts enter the case from the distribution block and is connected to the normally closed side of the reed switch and the common pole of the relay. When the relay is engaged, the fans turn on and power is applied through the diode to the power conditioner and then to the ATX PSU and the display (next). The power conditioner was required to keep "fan noise" out of the display. The original concept behind the capacitor was to keep the machine from rebooting when the truck is started. Unfortunately, I guess it is not big enough so now its job is for extra filtering before the conditioner.

Here is the complete assembly.