the shear tool in illustrator will allow you to creat a parallagram from your rectangular sign. In an isometric perspective view where the vertical height of the back wall is equal to the front wall... this tool will be enough, but in a true perspective view where the foreshortening will make the back wall height appear shorter... you can use the shear tool to make the horizontal lines in your sign match either the top, or the bottom of the building, & if the vertical lines need shearing too (they wouldn't in your post above, but may need a little in an imperfect photo)... then shear them with the same tool... then to get the horizontal edges to converge, use the "free distort" tool under the "effests" menu. You should also tweak the aspect ratio of your rectangular sign before you do the other tweaks mentioned above... but this could be misleading, because a rectangular sign may need to look square to be "correct"... but the typical customer (at least the ones I usually get) will not understand that.
but I would get a photo of a straight on shot of the building before I would spend too much time doing the pseudo perspective, because if it's too far off to the side