  We prove a reducibility result for a linear Klein-Gordon equation with a
quasi-periodic driving on a compact interval with Dirichlet boundary
conditions. No assumptions are made on the size of the driving, however we
require it to be fast oscillating. In particular, provided that the external
frequency is sufficiently large and chosen from a Cantor set of large measure,
the original equation is conjugated to a time independent, diagonal one. We
achieve this result in two steps. First, we perform a preliminary
transformation, adapted to fast oscillating systems, which puts the original
equation in a perturbative setting. Then we show that this new equation can be
put to constant coefficients by applying a KAM reducibility scheme, whose
convergence requires a new type of Melnikov conditions.
