Basically you need power wire (thick one 4GA or 2GA prefer) running from the battery to the back (where the amp and PC/PSU will be) to a distribution block. From the Distribution block, split it out to 2: 1 for the amp and the other for the PSU (Opus). The wire from distribution block to the devices (amp/psu) doesn't have to be as thick as the one from the battery.
Also you need a short and thick wire for GROUND. How long this wire is depend on where the amp and PSU is to the bare metal part of the car that you can bolt to.
Now you need at least, a VGA extension cable from the PC to the dash where the lcd going to be. 2 USB extension cable. One for USB CD/DVD ROM, the other you may hook to a usb hub for other usb devices (TS, GPS, MOUSE, .... )
Now if you are powering the LCD off the OPUS, you need a Power/Ground wire from the PSU to the LCD. The same for the CD/DVD ROM.
That's the basic idea. You can search the Show off your project forum for other people installation diagram and stuff.