In a bowl, stir together the sour cream, 2 tsp. garlic and 11/2 tsp. chili powder; season with salt and pepper.
In a large bowl, combine 2/3 cup EVOO with the remaining garlic and chili powder, 1/2 tsp. salt and 1/4 tsp. pepper. Add the mushrooms and eggplant; toss to coat. Thread onto skewers with the tomatoes.
Heat a grill or grill pan over medium heat and brush with the remaining 1 tbsp. EVOO. Grill the kebabs until tender, about 10 minutes.
Meanwhile, in a saucepan, cook the couscous according to package directions. Serve with the kebabs and sauce.