Hey gamegurl!
Did you kill the guys who murdered the poor merchants with the broken whell on their "lorry" <img src="/ubbthreads/images/graemlins/delight.gif" alt="" />? If yes look if you have got some evidence in your Inventory...
...if you don´t have take a look at the cave again and search for one...
Speak with Marcus, Tingalf, and so on again.
What makes me a bit confused is that Tingalfs Gang isn´t in the sewers before you get the quest and the leader of the gang doesn´t become hostile until you made something specific in this quest... <img src="/ubbthreads/images/graemlins/confused.gif" alt="" />