It could be built into the Computer.
Look athe wiring harness for the engine sensors. There could be a tiny vacuum hose in it. That will go to the computer as the sensor could be built in. Those early years were a formative bunch for GM as they didnt have it all together, so it can be a guessing game.
Most of the time the sensor is underhood.Many years they were mounted on the air cleaner housing
Fact is stranger than Fiction