Call the Banners (2) Tweek | Walkthrough South Park: TSoT Guide

You can find Tweek in his parents' coffee shop

You can find Tweek in Tweek Bros. Coffee. Head to the back of the coffee shop. As it turns out, he cannot join the quest until he finishes all his duties. Head to Kenny's house to help him.

Enter through the garage door

Kenny's mom gives you the key to the rented room at the back of the garage. Get inside. Once there, you will have a battle to fight.

It is worth to carry some boosts and health potions, since the fight might be a bit difficult. First, focus on the woman, who can perform multiple attacks at the same time, so it is easy to fail to block them. Butters' Hammer of Storms is especially useful here - this can damage both her and the other opponent standing next to her.

Climb up the ladder and hit the plank hanging down from the ceiling. It will fall down and create a passage, as shown at the screenshot. You can find the delivery for Tweek on the shelf. Turn off the burner so you could exit and head back to the coffee shop to complete the quest. As a reward Tweek gives you the key to his chest, which is located at the back of the building. You can get to it using the ladder.