Having fitted the mirrors from a 2003 discovery to a 1990 discovery I have found that
The mounting holes are different, but they can be drilled out to make it fit.
The mirror pan and tilt movement is different but can be wired up by using a DPDT relay and tucking it in the mirror mounting triangular recess.
The power fold can be wired to a DPDT 3 position toggle switch or via a relay to the ignition circuit.
I have to say, WHAT A LOT OF WORK ! I wish I had got the older mirrors !
Peter (Mad on Gadgets)