Steve;

You haven't given nearly enough info with which to help, but given his knowledge and experience, Eric probably has been able to point to a common source of headlight problems.

Referring to wiring diagram (http://www.sw-em.com/1800%20Wiring%20Diagram.jpg), at footswitch check for Headlights power at Blue wire (incoming power from Light Switch), then Blue-Red and Blue-White, alternately when activating footswitch. If no input, check upstream (Light Switch and associated wiring), if no output, rebuild/replace footswitch.

Good Hunting!

...and incidentally, the internals of Light, Wiper and Fan Switches of the factory 1800 wiring diagrams are not as detailed (or even correct!) as those of the 122 wiring diagrams (it wouldn't surprise me if these incorrect wiring diagrams came from Lucas, in keeping with their God-awful general engineering and materials philosophy)...so beware when troubleshooting using the Switch internal connections indicated...I am in the process of generating corrected diagrams...but this shouldn't stop you from finding and remedying your issue.