this is a nice straight forward 1st project, most of the complaints with the heater on a 90 is due to the foam packing around the sides of the matrix, it ages badly and falls away leaving gaps for the air to bypass the matrix.
once you have removed the heater assy drill out the rivets around the assy to remove the matrix, if the matrix is leaking or damaged you can get thicker matrix cores as an upgrade, remember to use a good stiff foam to pack the sides of the matrix.
also it is worth buying the foam packing piece that goes between black plastic wing manifold & the heater assy.
also check the "birdsmouth" drain valve.
I drilled a 5mm hole in the bottom of the fan chamber to allow water to drain out as i have filled the fan with water a few times