I had a very similar problem with our 1992 Olds 88. Problem turned out to be a common splice point shared by the rear defroster and the interior lights. Apparently, the wiring was undersized from the factory and after X number of uses, the wiring deteriorated and since it began pulling too much current, it overheated the circuit, tripping a thermal circuit breaker. After the rear defroster timed off, the circuit & circuit breaker cooled down and the interior lights returned to normal operation.
Service facility installed an updated wiring kit and problem solved. Been fine ever since.
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