I have had this problem on two of my 740s'. I traced it to poor grounds out on the inner fenders. Cleaning them up will help keep this from happening again. The connectors on the lamps probably could stand a scubbin' also.

If you really want to FIX this problem, make or a buy harness with relays that removes the load from the internal wiring/switch(See Sterns' Lighting website). Then the internal relays only have to power up the HD relays under hood. I have made up two harnesses (so far) for our fleet and it is one of the best improvments you can make lightwise IMHO. Instead of 12.6 volts at the lamp, I now have .05 volt less (13.70) than running volts(13.75) at the lamp. BIG improvment in light output,even with halogen sealed beams.
I bought my supplies (wire, relays, circuit breakers,etc, from WAYTEK wire.(www.waytekwire.com) Good pricing on Bosch relays ($2.61ea)and much better wire than you can buy at the chain auto stores.

Anyway, hope you get your lights back on soon.
Later, rcs