A very slow water drip at central aircondition control unit below the contactors and fan when heating is turned ON

Water stain at central aircondition control unit below contactors and fan when heating is turned ON

Furnace? It sounds like you may have a condensing furnace. They only make these on inside (split) units because they can't figure out what to do with the condensate water on outside applications.
On a condensing furnace, the extra heat is extracted from the exhaust by condensing the water vapor out of the exhaust gas. The exhaust gas then is cool enough to exit through pvc pipe instead of metal. This is the best way to know if it is a condensing furnace. (pvc exhaust piping)

Seal the leak with silicone caulk designed for high temp or tighten plumbing connections if water is seeping out. If it is condensing from outside air, insulate or isolate from outside moist air.