It's been ages that looking for these solo female artist I forgot the song, I only remembered the music video like taken a movie or something a teenage movie she had a seen singing in the backseat of the car. The singer looks like one in M2M and plays guitar.
I think of Alanis Morissette's Ironic.
We can see her several times in the backseat of the car.
And she's brune, has a long hair, so you can find her a resemblance to one of the M2M girls.
Perhaps you are searching for Don't Hold Your Breath by Nicole Scherzinger? It features a solo female artist and she is singing in the backseat of a car for some part of the video.