> I think part of my problem here is that I don't have a solid understanding
> of what the difference is between impedance and resistance. I'll have to
> study this up more. 20+ years ago in college, the extent of my electrical
> experience was EE. TTL circuitry and the like----wiring up flip flops and
> all that. The analog universe I've learned on my own, which leaves
> significant holes in my understanding.

Well, your background is better than most -- looking at both your and
Clarence post, I am assuming we all have an EE background?

Remember that the voltage drop across a wire (or chassis) is in direct
proportion to the current being drawn. So if you are grounding something in
common with a turn light, the drop you see across that path will be seen by
whatever is on that line. So, the supply voltage to your newly installed
device will modulate slightly and that could cause problems, depending on
what the type of device is. That's why it is good engineering practice to
bring your ground together at one point, close to the supply in most cases.
The signals are not electrically isolated but the current flows are kept
separate.

For your purposes, Clarence suggestion of using that huge bar is very
legitimate.

Regards,
Remco