Weak headklights on 00 Ford Taurus- What to check?

The problem is probably related to bad connections going to the reg headlights , even a dirty or corroded etc fuse connection or relay connection or switch can cause such problems.

You had mentioned you bought new headlight bulbs if you still have them they might come in handy for a test if not you could get extra headlight bulbs from a U pick auto wreckers and also clip off both headlight socket plug wires as long as possible.

Once you've the extra bulbs and sockets you could try running them to the battery posts and see if the lights look any brighter, basically the centre plug wire is the negative on the dual bulb head light sockets.

I suspect that the vehicles whole electrical and charging system probably has corrosion or rust or bad connections running from the fuse box , power junctions , battery , alternator , starter , engine block ground posts and other grounds etc.

Hope that helps and best of luck.BY the way both of the headlights ground wires probably connect to the same ground junction and I'd start there , maybe make up a whole new ground connection, new ground wire same gage or thickness and use the proper o-ring connectors and wire connectors.

More than likely is going to be in your headlight switch or dimmer switch. Hope this helps.

th elight in the headlight

are your headlights foggy/yellowish?