I have a 2006 ford fiesta 2 door, when I press the lock button on the key to lock the car, both doors lock and unlocks same time. When I disconnect the central locking pin from the drivers door, I can lock and unlock the car with no problem with the key but the moment I put the plug back on the doors cannot lock even with the key on the door.

I would really like some help with this because I have bought new battery for the keys, I have replaced the door mechanism, I even bought new harness. 

Sounds like door locks to me, more than likely drivers side. 

I had an issue with my 08 Fiesta shortly before i sold it and was told that it could well be a fault with a door switch which would need replacing (no idea how easy it is to get it sorted as i sold the car).  Do you get the 'Driver door open' warning come on? mine was pretty temperamental and would go off when i was driving down the road causing interior light to go on but more frustratingly was trying to get the damn thing locked when i parked it anywhere. 

I have exactly this problem with wifes 2008 mk6. 64k on clock from new. Intermittent problem Locks but unlocks all doors. Latest is message saying drivers door is open, again intermittent. I tried giving the main harness to drivers door a wriggle and I thought this cured it for a while but this may have been a coincidence. 

Anyone have a fix for this.  thanks.

A tiny update. I tried the multi connector door to car and the pins are spotless no corrosion. I seldom drive this car but today the drivers door OPEN light was on constantly.

The car was difficult to lock Locking but Opening  on same cycle. Closed the door quite hard and problems both went away, light out inside and locked from outside.

This may well be the lock itself. Looks difficult to change by some of the videos I have watched.