To prepare the pork, combine the first 9 ingredients in a large zip-top plastic bag; seal. Marinate in the refrigerator 24 hours, turning occasionally.
Remove pork from bag; discard marinade. Sprinkle pork with 3/4 teaspoon salt and black pepper. Place pork on a grill rack coated with cooking spray. Grill 18 minutes or until a thermometer registers 155° (slightly pink), turning occasionally. Let stand 10 minutes before slicing.
To prepare relish, place onion and bell peppers on grill rack coated with cooking spray. Grill onion 4 minutes on each side or until tender. Grill peppers 12 minutes or until blackened, turning occasionally. Place peppers in a large zip-top plastic bag; seal. Let stand 10 minutes. Peel peppers; cut in half lengthwise. Discard seeds and membranes. Chop onion and peppers. Combine onion, peppers, basil, and remaining ingredients in a medium bowl. Serve the relish with pork.