I will install a laptop carpc in a bmw e46.
the sound will go thru the HU, the fuse will secure the laptop, usb for gps module, mobile phone and maybe a 3rd usb power socket....
"case": Skoda Superb Elegance 1.8T
Asrock G41MH-GE, E2180 2GHz dualcore, 1024MB DDR2/800, Samsung F1 750GB/7200RPM/32MB, M4-ATX, Lilliput FA1011 HDMI touchscreen, ASUS Xonar DG PCI, homemade Quectel L10 USB GPS
will you be adding USB hubs or just using the laptop's onboard USB ports?
if using the laptop's USB then you only need to to know the laptop charger/PSU power draw. if using powered USB hubs then add the draw of the laptop and hub power supplies and round up to the next highest (standard) fuse rating. for example, if your laptop PSU draws 5A and your hub draws 2A then a standard 10A fuse would be fine.
remember, the primary putpose of an power/in-line fuse is to protect the wire. so make sure your power and ground wires can handle the amperage that the fuse is rated for. you can always downsize a fuse, but never use a fuse rated higher than the ampacity of the wire you use. if you use smaller wires that branch off to each component then you need to fuse them separately as well (again, not to exceed the ampacity of each wire).