66 dart with no lights

^^This and treat each problem as if they are separate because they just might be.

For example, the headlight power ONLY (not park or tail) comes directly off the ammeter circuit to the light switch. There is no fuse, except the main fuse link, and a breaker built into the light switch.

Go here to download a 66 Plymouth manual for free. The site does not have a Dodge manual, but the Plymouth one will be essentially the same

https://mymopar.com/service-manuals/
Also on the same site are 2 page aftermarket wiring diagrams which are sometimes easier to follow, but which leave out some items. Unfortunately you have to screw with zipped files, but they ARE free

https://mymopar.com/wiring/
The "little red light" is your high beam indicator, and may show that you are getting power to and through the light switch AND the dimmer switch. This means that you may have bad connections in the bulkhead connector, a bad ground out on the radiator support for the headlight harness, or even both lamps blown out.