Normally you would drop the headlight switch and let it hang. Then check to see if the dash light terminal had 12V as a first step. Maybe the new switch will be the ticket. If you have problems after install, report them. Sometimes an aftermarket headlight switch is slightly different as to wiring. Did you buy the 12V version?

As for anything else to check, you only mention one other issue. The fuse block on firewall. You would use an ice pick voltage tester for that. You would test input side of fuse and output side of fuse with ignition switch off and ignition switch on. Report results and report what each fuse does and what item it is protecting.

You are not answering my questions from my last post.

To answer your other questions:

With mechanical gauges, there are not many wires to need to buy a "dash harness".
Adding turn signals may require socket changes and a few wires. Dual filament bulbs are required which need dual filament sockets.
The decision to buy a whole new harness depends on the condition of wires. Ratty or decent. Stiff or flexible. Loose terminals on the ends or tight. There are some very good advantages to a new harness.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn