I had to write the whole sequence, these 2 quests are busted easily
1. Get black market quest from Trevor/merchant guildmaster/Verdistis and missing wagon quest from Marcus/merchant/Ars
conversation with Trevor to get the quest2. Find wagon, talk with Luther the wagoner and report to Marcus
3. Return with wheel, look for clues first following a blood trail then a trail of coins (find corpses, cave, fight, TAKE letter)
the loot cave, after you've found the burning wagon, look here4. Report which clues(with letter) you found to Marcus - get a clue to talk with Tingalf.
5. Tingalf says in a speech option : new lair under my feet
6. Go into the Ars sewers (NE corner), fight, TAKE letter (and the nifty 2-hander Singing Blade)from the corpse
Ars sewers, look into the NE corner7. Go back to Lt. Robin/Ars and report, 2 speech options, use both
Ars Magicana, Nr. one is Robin, look here
Robin's speech options, look here8. Report to Marcus and later on to Trevor.
Final report to TrevorKiya