Using Host as a Wire

It is not uncommon to use the 'chassis' as a power plane, regardless if you are working with a car or a laser. As long as it's all electrically interconnected metal this will work fine.

As for the example of fog lights: in some cars they are mounted in the plastic bumper and need a ground connection. You could run that to the nearest metal chassis point, but perhaps some people choose to run in all the way back to the battery.

With things that rust the chassis connection isn't always that reliable though. I've seen plenty of older cars where the red tail light lost brightness as the indicator blinks (european cars have orage indicators and separate red tail lights that are always on). Contact resistances can really be a problem if things corrode.

With laser hosts corrosion isn't that much of a problem though and carrying the negative through the case is generally fine. One thing to be aware of is mechanical connections between aluminium and copper though: even if mechanically solid they are prone to corrosion and increased contact resistance.

This is the reason why in mains power application grid aluminium wires are connected to domestic copper wires using special connectors.

yea I get what your saying with these new plastic cars. I am also from the northwest where it takes a long time for cars to rust enough to cause a ground issue. I was more so speaking of the people that should not be touching cars... not even the steering wheel. Ya know the ones that do their own "tune up" and wrap the plug wires around the exhaust manifold. or cram a t-shirt in the airbox as an "air filter".
Back on track. I did have a few laser builds with the 501b hosts that became a huge pain getting the ground circuit complete because of the anodizing everywhere.

Anodizing will definitely cause problems when you want to conduct electricity.

Most flashlight hosts will have un-anodized threads connecting the pieces which conduct fairly well with some mechanical pressure. If they anodized the threads too though, chances are your connections will be very poor even with a lot of mechanical force on them.

The best way to check is to just run a large current and see what the voltage drop is. With a current limited power supply push 5 amps between the battery terminal and the point you take power from. If that results in over (half) a volt you probably have a problem.

Another thing to be aware of are the tail clickly switches on some hosts: sometimes the switches used are so bad they actually are the limiting factor and will burn out if your laser runs more current than the flashlight was designed for.