If you are getting water in the passenger foot well (of a RHD car) there is probably a problem with the heater matrix. It's not the only possibility but that's what I'm dealing with today.
First up remove the two heater hoses on the fire wall. Pull the wire retainer out of the hose plug and pull the hoses off. Put the wire retainers back into the the plugs. The plugs will snap back onto the outlets with the wire retainers in place.
Try and keep the hose ends as high as possible otherwise the coolant from the engine will run out!
Remove the torx screw from the center of the metal cover.
Remove the metal plate. Remove the rubber gasket.
At this point use a length of vinyl tube and try to siphon out as much coolant out of the outlets as possible.
The heater matrix is located on the passenger side of the center console.
Remove the electrical connector from the heater feeder tubes. Pry the tang away from the connector on the back side of the connector and slide the connector down and off the mounting clip.
There are two potential areas that the coolant could leak out. I have labeled them drip #1 and drip #2.
Drip #1. There are 2 o-rings that form seals between the inlet tubes and the heater matrix. If you get a drip from here you only need to remove the inlet tubes and change the o-rings.
Drip #2. If you get a leak from here you will need to replace the Heater matrix. Just make sure the drip is coming from the bottom of the distribution box and not dribbling down from the junction of the heater and the inlet tubes.
When you separate the inlet tubes from the heater, you will get some coolant run out and try to make a mess. Arrange something to catch the spill.
Remove the bolt that clamps the inlet tubes to the heater matrix. Near the top of the inlet tubes is a torx head screw securing the inlet tubes to the air distribution box.
At the very top of the inlet tubes you will notice that there is about a centimeter gap between the firewall and the flange on the end of the inlet tubes. This space is taken up by a foam gasket. This gap is so that the inlet tubes can be separated from the heater in situ.
Pry the tubes from the heater upward and toward the front of the car. The tubes can then be removed. You will see the o-rings attached to the ends of the tubes. If they are your problem, replace them. don't forget to lube the o rings with something like petroleum jelly or rubber lubricant.
If you need to replace the heater, undo the 2 torx screws at the top and bottom of the heater mounting flange. Pull the heater out of the air distribution box. It's a tight fit but persist.
I don't know much about the diesel variant. I believe they have an electric "auxiliary" heater as well as the coolant type? Sorry but I can't tell you anything about that.
