What's the crack with this? Sunroof or heater matrix or.what?

search - theres loads!

odds are it the scuttle panel drains behind the front wheel arches are clogged with dirt/leaves etc.

it fills up the space with water and it flows in through the grommet from the bonnet release cable.

pour water down the windscreen, it should come out down by the wheel arches. if not water comes out, get the arch liner off and start diggin out the compost

 

generally the sunroof causes wet rear foot wells.

Not sure if crud inside the wheel arch liners can cause it? See this thread:

http://www.briskoda.net/forums/topic/205761-check-your-wheel-arch-liners-now/

 

Probably worth cleaning them out either way.

There's a sunroof drainpipe in the A pillar somewhere can cause problems if it gets blocked I believe. Also worth checking the door membrane, the slightest drip can cause a world of wetness after heavy rain.