Trevor,
I found this in a book I have.
1 cup beef broth, 2 tblsp balsamic vinegar, 2 tblsp canola oil, 2 tblsp minced basil leaves, 1 tblsp dijon mustard, 2 cloves pressed garlic, 1 teaspoon sea salt, 1 teaspoon black pepper. Mix it together in a bowl. Put the tri - tip in a baking dish or plastic bag with marinade overnight or at least 4 hrs. Then remove tri-tip and pat dry, then rub in 1 tablespoon of lemon zest (grated lemon peel) on both sides. Mix 1 1/2 teaspoons ground cumin, 1 teaspoon sea salt, 1 teaspoon black pepper together and sprinkle over tri-tip.
Hope this helps. If you try it let me know how it turns out.
Brooks