Prepare a hot fire by piling charcoal on one side of grill, leaving other side empty. (For gas grills, light only one side of grill.)
Remove heavy outer husks from corn; pull back inner husks. Remove and discard silks. Tie inner husks together with string.
Stir together mayonnaise and sour cream in a small bowl, and set aside. Place Parmesan cheese, chili powder, and lime wedges in small serving bowls, and set aside.
Coat each corn cob lightly with cooking spray. Sprinkle corn evenly with salt and pepper. Position corn on food grate of grill so that tied husks lie on unlit side to prevent burning husks.
Grill corn, covered with grill lid, over medium-high heat (350°to 400°) 10 minutes or until golden brown, turning occasionally. Place grilled corn cobs on a platter.
Spread corn evenly with mayonnaise mixture, and sprinkle evenly with cheese and chili powder. Squeeze lime wedges over corn.