Actually hardware consists of ALL physical components. In a vehicle PC system, you could possibly have any or all of the following hardware installed:

PC Power Supply
PC Motherboard
PC RAM
PC Hard Drive
PC Optical Drive
Relays and/or control boards (like Phidgits or Fusion Brain)
Sensors (for TPMS or OBD)
Head Unit
Amplifier(s)
GPS unit
iPod or other MP3 player
Any/all Wiring