Depending on how yours is wired it may not have a jbox. Some have the Main harness from the engine goes straight to the gauge with a pigtail for the system link gauges. Check all your wire connections. Try unplugging each link gauge one at a time and see if the issues go away.
I know it isn’t a current solution. But Searay has an outside vendor that has built replacement kits for these obsolete systems. I have used the demos and they are very nice. They come as turn key systems that replace the original switch panels on the dash and modules. They will only be...