TexasSwede is right.
The CB (circuit breaker) inside a Mustang/Shelby/Boss headlight switch can "trip" when overheated but will reset once cooled off. I assume you can "blow" one from constant abuse or maybe just age and then, a new one is in order... I learned a lot while troubleshooting lighting gremlins in a 70 while swapping with 69 parts.
Got this CB info straight from the Ford Shop Manuals for Mustangs
Year Amps
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66 12 (Falcon also) 18= Comet, Fairlane
67 ? No manual
68 12 (Falcon also) 18= Cougar, Fairlane, Montego
69 18 (also Cougar, Fairlane, Falcon, Montego)
70 12 Falcon/Maverick also 18= Cougar, Fairlane, Montego
69 CB rating IS higher than the 70 (due to supporting 4 headlights vs 2) but is exactly plug & play.
FYI: there are multiple CBs in each Headlight switch & not only for lighting, like horns!
HTH,
Phil