What can be done about condensation in 1999 Lincoln Town Car headlight?

About the only thing you can do is replace it. It should be completely sealed and therfore not get condensation, but if it has a crack or hole it will cause this.

Ensure the cap for the bulbs are installed correctly and not allowing water to enter.

If no area are found to be leaking and the headlighta sre in good condition without cracks, you can try applying some sealant along the joints on top of headlight between the lens and housing to prevent leaking.