Someone asked for a HOWTO/Walkthrough so thought I'd do one.
Firstly to check if your matrix is working as it should (sort of a flow check):
In the below image you will see red arrows indicating which screws to undo.
undo the screws that secure the under panel (black plastic) and remove the panel. the steering column under cover can be removed aswell to make access easier, this is done by simply pulling it out and upwards (clips hold it in place and are easily pulled out of their sockets).
Once you have removed those panels undo the screw that holds the black plastic footwell vent onto the side of the matrix housing. once removed you can simply slide the carpeted panel down and out.
you should now have good access to the matrix and pipework as below:
it's easy to check if you heater matrix is getting the flow and working correctly now.
We all know how a radiator works, hot water flows in, heats the radiator and heat comes from it by convection/radiation depending on the type. For hot water to flow in water must flow through and out the other end, otherwise the water backs up until it reaches the same pressure as the rest of the system but goes nowhere.
Now, with the engine at normal operating temperature and the heater fans SWITCHED OFF. BOTH pipes should be as hot as each other, because the fans aren't moving air through the fins to cool them so no heat (or very little) is lost through the matrix. In this case your matrix is highly likely to be working correctly (within reason).
If BOTH pipes are COLD then this could mean 1 of 2 things. 1. your heater may have an airlock (though it's unlikely by the design of the water system on the 75/ZT) but in any case it's worth checking it. 2. If it isn't an airlock your matrix is sufficiently blocked so that it allows NO FLOW whatsoever through the matrix and requires removal and specialist attention (see how to remove below).
If 1 of the pipes is HOT and the other is barely warm then there is a substantial obstruction in the matrix but it is allowing SOME flow through it (usually the passenger side is blocked but the driver's side is flowing as it should).
It is a waste of time putting a hose pipe onto the pipe to try and "flush" the blockage through. Why? Because water ALWAYS takes the route of least resistance. If 6 out of the many pipes are blocked the water will simply flow around the blockage (it CANNOT flow THROUGH it). You may dislodge some of the crud/scale that is in the matrix but it will ONLY be from the pipes that are still allowing flow through them.
Now to the HOW TO part:
Do as above & remove the panels to gain access to the matrix and pipes.
Make sure to remove all the coolant from your system before removing the matrix.
You will see in the image below an arrow next to a piece of black "sponge-like" insulation, this covers the air conditioning pipes from heat soak. Behind this black insulation is a bracket securing the aluminium pipes to the matrix, remove the screw and bracket. You can do so by going behind the clutch pedal with a phillips screwdriver (fiddly but easy enough).
Now you have done that it's time to remove the matrix. The image below shows the pipe collars that hold the main pipes to the matrix. Have something ready to catch the water that is left in them. Undo the screws and pull the collars to one side (you can leave them on the pipes). Pull the pipes apart enough to let the water drain out. once the water has drained and your ready to remove the matrix you can release the pipes completely by pulling the right hand side pipes further right - they will move and maybe even bend slightly but don't worry they'll go back together easily enough. You can turn the pipes that go into the matrix slightly to make it easier - you won't damage them unless your rough.
Now that the pipes are free you can simply get hold of the matrix and slide it out towards you, don't twist it as it will jam.
Once you have it removed take it to a radiator specialist and they will clean it properly.
Hope this helps