The relay, as Brookie says, is above your feet behind the panel. It is a double size rectangular relay. It is fiddly, but possible to unclip the casing on the relay, then you could try blasting it with a can of electrical contact cleaner? This did not help my problem, but did not do any harm either! turned out to be a duff switch..
I always thought the plenum was the thick part of the inlet manifold that the intake runners branch off? The blower duct is also a manifold, so maybe a 'techie' term??