The 5th pin in the glow relay is for a coolant temp sensor. It goes in the head tucked behind the IP. If the 350 doesn't have a BFS, there should be a spot ahead of the coolant thermostat where the 300 has the BFS 50*C switch. The glow relay temp sensor is $12-15. Depending on how the harnesses mesh, the 50*C switch wire should end close to the glow relay anyway.

Sixto
87 300D