Hi
I've got a same year transporter, I've never replaced the heater matrix but i had a quick look on my van and it looks as if it would be a nightmare of a job,I think it can be done without removing the whole dash but you will need to remove the glove box and centre consel, an awkward job cause you will need to lie on your back in the footwells. The matrix seems to be located against the bulkhead.
sorry this can be the only assistance i can give you.
Good luck
colin
January 2008