I'd check the charging system. 99 times out of 100 headlights go dim because the alternator is weak or the regulator is about to puke. Assuminig everything else is functioning properly i.e. wiring and connections, the charging system is the culprit. Turn signal oscillation slows down...interior lights flicker. The junction block on the radiator support connection may have a deteriorating contact--rare--but it happens.
The oldest test in the book is taking the positive cable off of the battery while the engine is running. Before integrated circuit altenators ( 1973-up ) this could be safely done and it would reliably pin the alternator or regulator as the likely problem. Hold a screwdriver to the case plug on the back of the alternator. If it is magnetically pulled to the alternator, the alternator is charging. That points to the regulator. Those old regulators used springs and coils to control the voltage and the points oxodize like ignition breaker points and they deteriorate.
Sorry to wax nostalgic, but us old guys pulled our youthful hair out back then when the electrical system acted up and we learned what to look for. Not saying this is definitely your problem, but when all else fails, go back to the very basics.
Good luck and I hope this helps.