I have been around South Park hunting for the underwear as a side quest...Yes underwear...I have 4 of 5 and cannot find the last one. Anyone know where I can find these sets of drawers? Pretty sure I found one in my own house if that helps any, along with at Cartman's house.
Underpants can be found in Kyle's Room, Cartman's Mom's Room, Kevin's room (the Star Trek Kid), Stan's room, and Stan's parents room (the one that most people miss).
Stan's house has a nonstandard floor plan. Across the hall from his sister, there is a 'hidden' door at the bottom of the screen. You'll find underpants in there.
This is related to the Friends in Strange Places achievement. Including that in your question and searches would more quickly give you the answers you seek. Per various sites, but sourcing trueachievements,
As per good RPG/Adventure strategy, loot everything as soon as you can. Take everything that isn't nailed down before coming back with a crowbar to get those. By adhering to this mantra, you will find things easier and, in this case, you should have collected all the underpants quest items already (which can't be sold so you wouldn't lose them).
To befriend the crab people for the achievement,