Basically, wiring "to a ground" is simpler, 'cause there is a ground almost anywhere close by; saves wire - in theory.. In alternating current work, it is bad juju to wire anything with a standing voltage on the device being operated; too much danger of a catastrophic short. But in a DC system, there is always a standing voltage, or "potential" on any device, but it may be either pos. or neg., so it really doesn't matter, and since an auto chassis and all the stuff attached directly to it constitutes the negative ground system, it usually is easier to wire "to the ground" by the shortest or easiest route. Personally, being an old sparky, and having worked with all sorts of systems in industry, I prefer to wire from the source, positive in DC or "Hot" phase wire in AC, through a fuse and switch, thence to the device to be operated, then to the return; neg. in DC, or Neutral or an opposing phase in AC systems. Saves an occasional brain cramp.