has been covered, but im in a nice mood.... okay I would say the main points are....
- How to power the system? Either DC-DC or an inverter ---> PSU. The latter is easier to build, but the former will/can be more effiecient. A DC-DC can also be pre-built (keypower etc) at a high cost, or built yourself (sproggy, etc) which means you need the know-how to do such a thing. Powering will be the bigger hurdles you need to decide on.
- Ground loops/buzzing. If you dont ground your components properly you will end up with buzzing in the output sound. This is one area alot of people dont take the time to do at the start, and then tear their hair out trying to fix it!
- How to run the output sound? Either Head Unit with AUX input/ direct car amp input (if you run amps) modification of an existing head unit, FM modulator input, tape adaptor are the main methods. The first three are most likely the best solutions, but can be a pitfall.
- Buliding the case. Make it strong (its in a car!) and able to fit all components. Also be consious of possible heat problems and mounting of hard disc drives to aviod excissive shock/damage.
- Fuse, Fuse, Fuse.. You must fuse the system in your car..... trust me on that one!
thats enuff from me...