Quick ground testing

Hi guys, Not new to electronics and have done a few installs of my own stereos and alarm systems, but have a quick question on testing the quality of my ground connections. I read the stellar article at (https://www.the12volt.com/installbay/forum_posts.asp?tid=49445) but have one question: across what span am I testing the ground? In other words, let's say I want to test the ground connection of my head unit. Where am I sticking the multimeter probes to get an accurate reading of the ground? It's unclear if one needs to go on the negative battery terminal and one on the sheet metal near where the ground wire is connected or some other variation. What's throwing me off is the reference to whether the probes are long enough. Thoughts?

What kind of problem are you having that you think you need to check ground of your radio?

with the headunit I have no current problem other than under-performance of the audio. That is likely something I will fix with running the constant power directly to the battery with an inline fuse at the battery but I was wanting to check to make sure I have good ground as well. Separately I am having issues with the dome lights because the switches are showing low voltage when it should be grounded which is telling me that the ground connection is not solid there. While I understand the basics of making a ground connection, I would like the peace of mind of having the resistance measurement, but I need clarity on where the probes need to go.

There should be virtually no resistance from chassis of the vehicle to the negative terminal of the battery. There should be virtually no resistance from the chassis of the inside the vehicle to the radio's ground wire, if of course your vehicle has a ground wire in the factory harness. Check from the case of the radio to the chassis of the vehicle. It sounds like your door switches are corroded. Not knowing what kind of vehicle you are dealing with, I can not tell you of the possibility of cleaning and lubricating the switches.

Thanks for the help. It's a '98 Lada Niva (Russian car). They sort of make the pinto look like a Ferrari... except they can get most places in the mountains better than a mountain goat. With that, it doesn't surprise me. Parts are dirt cheap, so I'll just replace the switches and re-sand the contacts.