A quick glance shows that it must be on the rear of the vehicle. It must also be illuminated by a white light and visible from 50' away. *HOWEVER* at least as of March 1 of this year, the lack of a light cannot be the sole stopping charge.
"B. Except as otherwise provided in this article or permitted by federal law, it shall be unlawful for any person to operate any motor vehicle on a highway with any sign, poster, colored or tinted film, sun-shading material, or other colored material on the windshield, front or rear side windows, or rear windows of such motor vehicle. This provision, however, shall not apply to any certificate or other paper required by law or permitted by the Superintendent to be placed on a motor vehicle's windshield or window."
Not sure if the temp plate falls under the listed exemptions. It wouldn't here. Like I said, common practice but not legal. A reason to stop, but few people would actually write it.