Start with knowing what they are and how they are set up. I was able to download the entire factory service manual thousands of pages, for my 1996 Dodge diesels. I suspect you can find a similar Chrysler factory manual or download for your trucks as well. You might start on the site to cater to the diesel folks, because they tend to last much longer and have many more numbers since the trucks hold their value pretty substantially.
I assume that the speedometer is electronic, not using a cable, but if it does use a cable and that’s probably your first thing to check.
Just act funny, especially if there are intermittent grounds or traces on the parent circuit board. Sometimes looking at the board will show you where you might have a supplementary ground that can help quite a bit with jumpy gauges. That said, if your gauges are not working at all, it starts to make one wonder if it is a huge, or a complete damage to a harness, or something else more severe.
How hard is it to pull the cluster and start looking at it from there? Sometimes if there’s not a tiltable steering wheel, it can really be a major hassle, especially if it has been stuck in there for the last decades.
