If you are doing extensive work inside a trunk of a car, it is actually easier to lie in it. Trust me. Now, as we can see from the picture, a big portion of what makes up the interior of the trunk has been taken out. The yellow could be washing liquid of some sorts. He uses an extensive amount of liquid and does not wish to wear wet clothes which would be the result of lieing in a wet trunk. He is probably reduced to wearing shorts only. The dog, heck its only chilling while being close to his flock/master which is not going anywhere anyway.
Or maybe he is just a zombie that was let out because the zombiedog who drove him home finally opened the trunk. Surprised and shocked by the first thing he saw he freaked and punched the nice zombiedog out cold. Now he will drink the homemade zombiejuice from the yellow can the nice dog brought him so he has strength to catch up with the Street View cameracar. Evidence of zombies must be destroyed!