Prepare the marinade: Combine the first 9 ingredients in a small bowl.
Prepare fajitas: Combine 1/3 cup marinade, onion, and steak in a large zip-top plastic bag; seal. Marinate in refrigerator 1 hour, turning occasionally. Remove steak mixture from bag; discard marinade.
Place a large nonstick skillet coated with cooking spray over medium-high heat until hot. Add the steak mixture, and stir-fry 5 minutes. Place the steak mixture in a large bowl, and keep warm.
Add bell pepper, squash, zucchini, corn, and remaining marinade to skillet; stir-fry 5 minutes or until vegetables are crisp-tender. Add to steak mixture; toss gently.
Warm tortillas according to package directions. Arrange 1 cup steak mixture, 1/3 cup tomato, and 1 teaspoon sour cream down center of each tortilla; roll up.