You don't need permissions to mention a brand name in a work of fiction. However, if you use a brand in a defamatory way, you could end up in trouble. You can't use a brand name in a title without permission, though. Also, you shouldn't use a trademarked brand in a generic way, as that dilutes the brand and will ruffle feathers.
Similarly with living people -- it's okay to incorporate them into works of fiction so long as you don't defame them. King had Jimmy Carter show up in The Dead Zone, but it was a benign sighting. If he'd turned Carter into an axe murderer, that would have been a problem. You can't defame the dead, however, so they are fair game (except nothing prevents a family from suing...even if they're bound to lose).