I don't marinade, or inject (unless I am messing around with competition techniques). But, if you simply must, use beef broth and the rub you are going to cook with. Just heat up a couple of cups of no salt or low salt beef broth, add in two tablespoons of rub. Cool completely and then marinate brisket in that overnight. It will add a little flavor, not much else. But, it won't screw anything up either.
Twas me, I would just rub it up tomorrow an hour before the cook.
me: I don't drink anymore
Yelonutz: me either, but, then again, I don't drink any less