Sure! There are three locations where black boxville can be located, one in each city. The easiest to find is in LS, near Ganton Gym. You have to enter the truck at night (21:00-7:00) and press CAPS LOCK.
Now cruise around the city and search for houses. If the building is accesible, you'll see the yellow marker upon it. Get into the house and search for valueables (TV, gaming console, VHS, stereo...) and press ENTER to pick them up. Then Take them outside and put them in your truck.
But make sure that you stay quiet (hold the sneak button) or you'll wake up the inhabitants of the house. If you do that, you'll have excately 10s to get out of the house or you'll get 3 stars wanted level and cops'll suround the house. So here's a little tip: Get a silenced pistol, and as soon as you enter the house, find the people inside (usually sleeping) and kill them. When you kill all of them, the sneaking marker will dissapear and now you can run aroud the house freely. Also, crouch and roll if sneaking to move faster.
When you finish your rampage, you'll have 5 minutes to take the truck to the garage (one in each city) and then you'll recieve money for valuables you stole (my record is about 5700$ in one run). If you steal a total of 10 000$ in burglary missions, you'll be rewarded with additional 3000$ and unlimited sprint (!).