Before you can start this side quest, you must have first completed two other sidequests, detailed here:
This side quest can be completed during chapters 2, 3, 4, 6, 7, 8, and 9.
On completing the Anna's Diary side quest, you will be given the Delivery Man Q side quest. After 4pm go to Quint's trailer, located behind the SWERY 65 bar, whilst he is home and look at the photograph on his shelf in order to get Quint to come away from the bike. It's a bit hit and miss so you might have to try a couple of times to get it to work, but what you are trying to do is get him away from his bike so that you can examine it and find something inside the fuel tank. After finding the item you can talk to Quint about it, which finishes this side quest.