Well... My Leatherman Wave has just about everything in it to fix a car but a socket wrench :P Especially with my bit assortment I got for it. Just wish it had replaceable blades on the wire cutters...
As far as automotive goes, like 90% of fasteners are 10,13,or 15mm, occasionally a 17mm. So some folding wrenches, or just a socket adapter would allow service to most more significant components. Combined with the cars Tire Iron, those few wrenches should allow you to service anything on the belt system. On newer, metric vehicles anyway.
Screwdrivers, flat, phillips #2,and T-20/25
Knife, pliers, wire cutters, a wire stripper is optional, since a knife can do it too... If you made the pliers locking, or 'robo-grip' style you might get a bit more versatility out of them...
A simple volt meter would not be remise.
The biggest thing would be to make sure the tools lock into place. A blade that can fold and cut your fingers if you're prying on something is a very bad thing.
