[mike_the_man]

I'm not sure if this will be any help, but I was having strange lighting problems on my 74. It turned out that my hazard light switch had fallen apart, and power for all of the marker and signal lights runs through it. Sometimes it would make contact, and sometimes it wouldn't. Took me a while to figure that one out. Also, make sure all of your grounds are good.

Trouble shooting electical is never fun. Good luck!