Random dim lights

One headlight dim vs the other sort of rules out the bulkhead connector they are both fed from the same source.

As mentioned above I would first check for good clean sockets.

Don't use sand paper as that removes the designed in corrosion protection on the contacts.

Get some spray contact cleaner, and some tube cleaner brushes.

Rubber erasers are good too.

Then the grounds and any connection along the way.

Dim equals resistance that is heating up a connection and eventually will lead to melted plastic or fire.

As for dash lights

If you turn the switch all the way till it clicks are the lights brighter?

There is a rheostat in the headlight switch and a sweep. If either have excess corrosion on them there will be too much resistance and dim lights