Ok first the case I originally wanted the super small in dash etc.. then I realized it would b underpowered and over priced so I instead opted for microatx.
That costs about $20 it is an enlight case.
I hacked the front 4 inches or so off, drilled holes to mount power supply and 2.5" HDD to the side. Note the safety glasses
Then I bought a lovely Neutrik NL4FC and the associated plate NEU-NL4MP this provided an easy way to connect power reliably thru the case in a manner that was tidy (not that you would know I care given the state of my car).
(A low profile heatsink on a mobile CPU is fine, but much smaller than I needed).
Now that the Case is all done I needed to pass a wire thru the firewall of the car.
First step was to decide where to pass it thru. There is a spot under the clutch and two on the far left of the drivers foot well.
I decided to use the ones on the far left b/c I didn't want to try and seal that gaping hole or cut a hole in the seal as I thought it might pop out.
The wire will pass thru the hole for the hood release cable as the electrical tape is nigh impossible to get off or on in the tight circumstances.
So I removed the fuse block circled in red.
There is the entrance to the tunnel for wires
I thought that it was an actual tunnel and tried to get a coat hangar from the inside of the car and out the green plastic access hole.
It was very frustrating. Then I unclipped the plastic wheel covers and peered up. Sure enough there was absolutely no tunnel at all.
My plastic wheel well liners are already kind of dinged up from driving on rough roads this last summer so I wasn't terrified of messing them up, otherwise you might want to be very careful.