This is found in the Night Gluttons level which is found over the Supermall in the Electro City Overworld map.
There are 6 total billboards to turn on. 4 of them are on the left of the level (before going through the grocery store). The second billboard only has 2 of its lights come on instead of 3, it still counts.
On the right side of the level, there are 2 more signs to light up, one of which is the parking sign.