Loss of dash lights

I have a 1991 Chev Pu. When I start the truck the radio lights up as always. If I turn on the headlights the radio lights go out and I also have no dash lights. The alternator gauge is also not showing anything. The tailights are working and all fuses under the dash are good. Is there a fuse somewhere else I am not aware of? Or is it something else?

The first thing to check is the positive battery terminal.

The battery in your vehicle has the side mount terminals. And corrosion can form under the red rubber cover for the positive terminal which can cause a voltage drop to the vehicles electricals. Remove the battery terminals and peel back the red rubber cover to expose the terminals. If corrosion is present that could be causing the problem.

Another area to check is the bulkhead electrical connector at the firewall for the fuse block. Water has been known to leak into the connector corroding the pins causing electrical problems.