Dave,

Had this situation in the 66 1800S. The high beam indicator bulb is located on the rear of the speedo unit (actually there are three of them) towards the bottom. If you lay on your back under the steering wheel the lowest bulb on the right is the high beam indicator. (center is turn signal and left is generator) You can pull the bulb out of the speedo and clean it with some glass cleaner or what ever. I found that a Q-Tip was not long enough to reach the blue lens, but if you go to your doctor he/she will have really cool Q-Tips about 8" long that work great. (May also be available at a pharmacy) I used de-natured alcohol on the swab and repeated until the swab came out clean. (About 20 swabs for all three lights)

The generator and turn signal can now be seen in normal daylight, but the dark blue high beam indicator is still a little hard to see during the day. No problems at night.

Hope this helps, and good luck.

Oliver
66 1800S