how do i remove heater inside coupe, toyota avensis 1998?

the heating system is not working in my toyota. I`ve tried to clean the cooling system. have changed cooling fluid. but that was no success.
now I`ve started to remove the panels around the heating regitry (?) inside the car to take it out. but this is a big task.
i beginning to worry about removing too many parts. not being able to asemble them again.
is it necessary to remove the upper dashbord, to get the heater out?
any tips on how to do this?