I am looking for a simple proof of the following theorem - Wasn't able to come up with one myself. Should be a use of the Bishop-Phelps theorem, in some way:

Let $X$ be a Banach space, D $\subset X^*$ a $w^*$-closed, convex set with nonempty interior. Then the $w^*$- support points of $D$ are dense in $\partial D$.

Note: The set of $w^*$- support points of D is defined as: {$f \in D$ : $\exists x_0 \in X, x_0\neq0$, s.t $f(x_0)=sup_{g\in D} g(x_0) $}. Obviously, this set is contained in $\partial D$.