Power is best to be taken directly from the battery, but it looked a bit difficult for me to wire through from the engine room, so I pulled power from the fuse box under the steering wheel, using plugs which replace fuse (it's also a fuse, but has a wire jumping out). My PC is not a monster super-woofer, it draws only 2A, it should be safe with those plugs.
I pulled a battery line (which is always on) and a line which is on when ignition is on, from fuse box to trunk, using 0.75mm2 (AWG 18) wires. Earth (ground) is hooked up to a bolt under the rear seat, which has already been used for earth of other equipments. These 3 power wires are bundled and a coupler is attached at the end.
For audio output, I used a converter which steals audio signal lines out of CD changer bus. It is designed and built myself before. (I tried using FM transmitter before the converter, but it sounded awfully.) This converter uses differential amplifier, formed by op-amps, at AUX input, to cancel ground-loop noise. To switch between CD and AUX (PC), CMOS analog switch is used to be controlled by PC.
EPIA's composite video output is hooked up to navigation system's video input. Quality can't be expected, because the signal is once converted to RGB to NTSC, and converted again to RGB at LCD side.
For control, a USB hub is placed between the front seats. A small gamepad and a keyboard are hooked up to it. If a cell phone is connected, it would become possible to check email or browse web in the car (currently impossible because no Linux driver is there). This setup is convenient because PC has only one USB connection, and still I can use many peripherals at front seat.
I put "hook" side (hard side) of Velcro tape on the base of PC case with double-stick rubber tape. It sticks well to the carpet inside trunk.
There is no protection for vibration or shock. (How the rubber tapes work for this?) Especially hard disk is sensitive to those. 2.5" notebook drives are more tolerant than normal desktop 3.5" drives, but also have smaller capacity and higher price. It would be interesting to use something like shock-absorbing gel, if I found a way to mount it with them.
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