Also have a look at this. The amp gauge and bulkhead disconnects are the first places to look. As you see in the diagram if there is a connection issue with the amp gauge it effects a lot of other things.

 

A note on Battery terminal cleaning, Don't pound on posts unless u want to buy another battery. I use a scraper instead of the brush-type post cleaner. You're looking for a really bright shine. If you use a scraper, be really careful of reducing the post diameter till the clamp won't clamp. And then you have to build a shim between the post and the clamp. Or a screw. Years ago, 1983ish, my CJ-5 had eventually 4 on each post.

Don't know if you can see, the right side of post is scraped, the left side is brushed.


If you have a gap between the clamp and post, this can work. the 2 screws on the blu are #10s, too large; the other on the post is a #6 sheet metal screw. Or even better a #4. And this can cut thru any corrosion that can be present, like the dark gray hard stuff that a brush won't touch. Most people won't like the screw, remember, it's for troubleshooting the problem!

 

I apologize for the out-of-place/time post. Need to pay more attention to what is going on.

The Power Probe type tester is nice cause it's powered from the battery full time. Mine is an Autel PowerScan tool, it can read resistance and 12v positive and 12vdc negitive. And A/C V like a PWM. The Power Probe was DOA, from that amazing place. The new PP might read the same as mine.
The test light has a +, - leds, plus a digital voltage readout.
The Innova 3340a has a few different taps.
I started with just a basic test light, and have a few others now.

Several types of voltage testers, it's just a little sprinkle. The power probe types usually have enough power cord to reach the rear lights. You can order an extra length of power cord for trucks or trailers.
The one on top is showing battery voltage and whether it is positive or negative.

Innova 3340a, showing off more complications.