[BackintheFold]

For the 1988 ur, and others, there is a contact in the door key switch that makes one wire positive or the other permanently.
Those two wires go back to the pump that has a central earth and two other connections. 12v on one and the pump sucks. When the door knobs are both down, the vacuum increases and pulls a physical switch in the pump. This stops the pump and sets it to wait for a signal from the other wire.

When the key is then turned to open, the positive goes down the other wire. Because the pump has moved a contact over, this makes the pump blow. When both door knobs are up, the excess pressure now flicks the pump's internal switch the other way and the pump stops. Again, the pump can now wait for a signal from the other wire.

There is no vacuum connection to the driver's door! So the whole "central locking" thing in an ur is just the driver's door operating a pump to open the passenger door...