I seem to remember posting a full explanation of the leaking heater box problem on here a while ago - it's pretty common (in all the FAQs etc), you guys spend too much time gas bagging and not enough time talking motors!!!
Still, it wouldn't be half as much fun on here without the banter
OK, a recap...
This is a pretty common problem, basically there is a hole in the forward bulkhead with a large plastic pipe inside the cabin connecting to a plastic box under the scuttle. The box is an air vent and the duct takes the air from the box to the heater unit, consequently the box is known as the heater box. The foam gasket is the source of the leak, they are pretty rubbish and a tricky job to replace, you are far better off sealing it with some silicone or similar - be sure to do the inside and then the outside too (as far as you can). HOT TIP! Use clear sillicone, and not the white stuff like i did (looks cr*p against my red paintwork!!
OK, some pix...
wet underlay caused by leaking heater box seal
Heater box on far right hand side of picture
Removing the heater duct
Note that MGR were aware of this and the TF has a completely different design of heater box - indeed it is more like a collection pipe and the seal is hugely better than on the F.
On the subject of water ingress into the cabin...
If you have an older car, or if you get caught out in the rain with the roof down reasonably often, then rain hitting the parcel shelf area soaks into the rear carpet and then the underlay behind, gravity and capilliery action draws the water downwards where it stays, against an unpainted panel! This is the only real rust spot on the cars so it is wise to be aware of it - subframes get a lot of surface corrosion but it's ntohing too serious (at least not yet) and the engine bay inspection panel (under the parcel shelf carpet) is also susceptible to rust. Here's what my 1996 F rear panel looked like in 2003 when i rebuilt her...
Rusty rear panel
Panel unbolted and removed (membrane to petrol tank enclosure behind)
Rusty inspection panel
Rubbed down, rust cured, primed & painted panels (look better eh?