The Mondeo has nothing in common with either the Scorpio or Sierra, except the Ford badge.

You'll be looking at making a Frankenstein's monster/cut'n'shut with the floorpan out of a Sierra/Granada/Scorpio and gluing it to the Mondeo shell to make a RWD Mondeo. Or, find a 4x4 Mondeo and do what Andy Rouse did with the first Touring Car Mondeos - disconnect the front wheels from the drivetrain.