Let $Y (N) $ be the moduli scheme of dimension two principally polarized Abelian schemes with level $N$. It is claimed in "G.Laumon - Fonctions zeta des variétés de Siegel" (Lemma 4.1) that to an algebraic representation $W$ of $GSp_{4}(\mathbb{Q})$ we can associate a $l$-adic smooth sheaf on $Y (N) [1/l] $.

Where can I find a proof of this please?