Preheat a gas (or charcoal grill), or a grill pan over high heat.
Cut four sheets of foil, about 10-12-inches long. Combine the shrimp, corn, bell peppers and onions in a large bowl. Add the olive oil, garlic, taco seasoning, brown sugar, lime juice, cilantro and cumin; gently toss to combine.
Divide shrimp mixture among the four pieces of foil, arranging the ingredients on the centre of each piece. Fold up all 4 sides of each foil packet, covering completely and sealing the packets closed.
Place foil packets on the grill and cook until just cooked through, about 6-8 minutes on one side. Flip, and continue to grill on the other side for a further 6 minutes, or until the corn is tender and shrimp are fully cooked. (If using the oven, bake in a preheated oven -- 400°F | 200°C -- on a baking sheet for 25-30 minutes, or until done).
While the packs are cooking, mix together the mayo, sour cream and garlic until thoroughly mixed through.
Once cooked, drizzle the corn with the crema; top with the cheese and chili powder; garnish packs with the cilantro (or parsley) and serve packets immediately with lime wedges, tortillas, rice, or other sides.