1965 C337 "Alternator not charging" light

[cessnadriver]

Hello ALL.
I have a 1965 C337, and the "Alternator not charging" light did not illuminate when BOTH alternator went off line!! I was flying last week, getting my CLT rating. When I looked down at the circuit breaker panel, because my G4 insight engine monitor showed voltage was down to about 20 volts, BOTH C/B popped! I had both landing and taxi lights ON, for about 15 minute, which I guess was too much load for the old girl's electrical system? SO checking the electrical schematics for the 1965 and newer Skymasters and found that the 1965-1967 do not have an alternator failure sensor. SO I ASK- how does the "alternator not charging" light get it power to show that one or both alternator are off-line? I've asked the Cessna Pilot Association Tech help but so far no word from them? Any assistance with this would be appreciated!!
Many thanks. BILLS