P38A P38 gearbox issue help

Can anybody tell me how the big black plug on the rear of the gearbox be disconnected and tied back out the way what do u think the problem is

Whem I took my engine and gearbox out last year I don't remember anything special. Incorporated into the plastic housing is a latch that you need to depress and then just pull them apart. It's on the top, or bottom.

They get full of mud and crud being under there though, so you could try cleaning any crap off before you start.