1 and 4 are grounds, 2 should have power and 3 only has power when the BCM gets a open hatch command from the switch in the lift gate handle

 

ground is good and the switch is corroded. I've tried to short the wire in the switch momentarily and the release actuator reacts very seldom maybe once in 20 trials. If the door ajar switch is defective will it affect the release of actuator?

 

The switch in the handle also gets corroded and will cause that issue. Pretty common.

 

finally my liftgate is working now. I replaced the lifgate switch ( it is very badly corroded ) located near the license plate and the one below the actuator ( I don't know what it's called ). I just took the replacement part from the junkyard. The old part that I replace has no continuity. Thank you very much tizzy 1 with the diagram you provided you give me an idea where to start it is really a big help for everybody.

 

I have an '05 with power lift. My exterior release switch works intermittently. Is that a tell tale sign of anything specific, like a bad release switch?

 

The switch that is on the rear hatch above the license plate gets very corroded, very common. Would definitely start there.