I use Express PCB And Schematic extensively for PCB design. Free, very easy to use. It is oriented to circuit board design but the Schematic program could be used for a car wiring diagram.
If you are looking for the basic functionality of Visio, check out Open Office Draw. Free, part of the Open Office Suite.
I think the website is
OpenOffice.org: Home but I'm not sure. Great alternative to Micro$oft.