That's true. Just be sure of the current needed before. Relay are great and that's the common choice in car. My own vision
  1. electric windows => usually less than 10 amp each windows => MOSFET
  2. heated windcreen => Yissh, more than often driven by a relay + thermal beaker in-line => RELAY OR, if you can, acces the switch and just send the pulse it need. Often you can us a simple transistor or PIC output directly as it send the signal to the relay or 'Heated windows' module. I did it often in the ast while i installed remote starter.
  3. headlights same as previous
  4. car radio => hard to say but MOSFET could be a smart choice to switch only the 'ignition wire'. That wire usually need less than 1 amp.
  5. computer ATX power supply => RELAY

If you can, access the switch directly and send the signal it need. This will use the original electric system. This will keep you away from electric modification and also make things easier/cheaper.

Well it was my own opinion based on my own car experience.

Hope this help.