Not to many details just stopped working only on / off light works. Someone told me problem was the neutral safety switch , it stopped working around the same time , it was 10 plus yrs. Ago. Need to know how it works so I can track down problem. Relay location,wiring maybe vacuum lines maybe neut. Safety switch. Or some diagram would help

Try replacing your coolant temp sensor first. If you haven't done a new T-stat in recent years, do that too.
Trans must see proper coolant temp.