Tutamun is in the cellar under Pierce's house in Verdistis. He's the wine merchant. The first time I went down there I covered the whole cellar and found nothing, but in the room to the east of the ladder there are barrels against the wall and when you move them, you can move through the arch there in the wall, it's a secret door, I guess. As soon as you move through the wall he speaks to you and engages you. I couldn't see him but I could still kill him.

Don't worry about the burning cart. You don't even need to lug that 500 lb. wheel around. Just report back to Marcus and he will give you the next part of the quest. You'll have to go into the cave to the east and into the sewers to get the dirt on Marco and Tingalf.