Car manufacturers realized the error of their ways as plastics became more prominent in car content. Now, all bulb sockets have their own ground wire. All the ground wires connect directly to the battery and not through the car body. Yes, the battery is connected to the body but very little current goes through it. Spark plugs still use the engine block for a conductor and so do a few other devices.
Classic cars are notorious for poor connections for headlights, horns, stop and tail lights in both the hot and ground sides.
Regarding the topic of a test wire with a lamp in series; if by accident your alligator clip touches ground as you probe around, it simply shines a light and no harm is done.
I use a sealed beam because it has standard 1/4" stabs. I can either connect using terminal connectors or a spare headlight socket. When the light shines, it's hard to miss and it won't burn up soft trim materials. - Dave