Check that the rear heat switch on the dash is not on, that transfers from cold air from top blowers to hot air from blowers at floor level.
Unless there is hot air coming from the top blowers, then I don't know why.
Sorry, can't offer any advice then. On my 03 CDI there are blowers in the roof that give the cold air. when I switch over to rear heat, they are disabled and the lower blowers then come on with heat. Not sure how your system works, but hope someone else can offer some advice.
HI, I have never seen one with covers/plates over the rear upper a/c vents? I also have a basic L model on an 04 plate and depending on what position the dash board switch is in we either get ice cold air from roof vents or hot air from the floor vents? Do you have the "RR" switch on the dash near the rear wash wipe button? If this is depressed we gethot lower! And not pressed we get cold form upper! Can you confirm that you do actually have air con? the snow flake style button to the left of the temp control dial?
Theres one thing I am trying today, but unsure if its just a recirculation valve or a hot air/cold air valve.
Take the cover off the off side in the boot, inside towards the front of this panel, is a small lever secured with a screw, loosen off, and push the lever up, and resecure.
Like I say, I do not know if this will work, only found it today whilst putting a accessory socket in the boot (for info, all the wiring is there, and working, just no socket, I got one from a breakers)
If it doesn't, then im afraid there is no cool air blower in the back, although, if you open the rear vent windows, you get quite a good, and pretty quiet air through flow.