To find Jonesy in the sewers, you'll first have to find the sewer entrance. It is a large hole in the ground, right in the middle of the road in the middle of the 3rd slope. Adjust your trajectory carefully and make sure you don't go too fast. Once you have fallen into the sewers, there is only one possible road and you will quickly reach Jonesy who is waiting for you behind a gate at the end of the route.
No need to activate something or stop in front of the sign, your challenge will be validated as soon as you have made your way to the sewers.
Written by Augustin "TheGreatReview" Héliot. Translated from French by Millenium.us.org.