If you have already exposed Tingalf, when you talk to Trevor in the merchant's guild and he asks you to look into a suspected black market dealer, you should just be able to tell him Tingalf has been arrested to complete the quest.
For the supply wagon, there is a faint trail of blood leading away. Follow it north west along the path until it curves north east and then straight east, to a cave (far east of the Dwarven Bread inn). You can drop the wheel.
If you need more hints, check out the webpage below. It doesn't sound like Tingalf has been arrested yet (you need to speak to Lt Robin), so hopefully if you explore the cave and talk to Marcus, Tingalf will be back so you can get a hint about the supply depot (you can head to the sewers early, but the main bad guy there will not talk to you if Tingalf has not given you the hint about the sewer).
Marcus-Tingalf-Trevor-Quest
