Headlights dim, not assembly?

headlight assemblies are all good. I have a 1987 Ford Thunderbird Turbo Coupe. New bulbs as well. Looked at the wire gauges and my fog light wires are thicker than my headlight wires, possibility? The bulb connection looks clean, no dirt, I'm not sure if it could be bad anyways? Checked battery output which is normal with car on, off and with my headlights on so I'm assuming battery and alternator are fine. It Could be a loose ground, but would there be other symptoms elsewhere? Any other ideas? PLEASE let me know!! Thanks in advance :)