I think in your specific case it would be up to the officer's discretion if you were pulled over for it. About 20 years ago I worked the night shift at a warehouse and my car's passenger side headlight went out on my way to work. I had time to stop at a parts dealer before they closed and picked up a new headlight, but I didn't have time to install it so I chucked it in the back seat and drove on to my job. I was pulled over right before pulling into work, and the trooper was reading me the riot act over the light. When I could get a word in I explained to him that it went out on me while driving, and I stopped to get one and planned to install it on my lunch break. I pointed to it in the back seat and showed him my receipt. He seemed pissed that his ranting was not necessary, but he didn't write me a ticket and instructed me to definitely install it that night because he'd be waiting for me on my trip home.

Perhaps I was lucky to get a cop who wasn't so much of an asshole that he'd write me a ticket despite seeing my best intentions to deal with a situation that could happen to anyone. Then again, I could have been pulled over by a guy who would have started off nice and thanked me for being conscientious enough to stop for the part that night.