Dim Lights

Well, the tailights don't go through the bulkhead connector. The power sourced from the alternator or battery does go through there, as well as all front lighting.

The ammeter being + or - depends on the alternator and its behavior AND the the battery condition (as well as the particualr ammeter's sensitivity); the ammeter is used to show the current flowing in and out of the battery (except for the starter solenoid and starter current from the battery).

You will be doing yourself a big favor to get that voltmeter and make some measurements; looking forward to seeing that info. We can be a lot more focused with advice if we have some solid data to go on. Without that, it is just general adivce to 'check and clean, check and clean'.....