YOU SAID:

riding shotgun with my hair undone in the front seat of his car

INTO JAPANESE

彼の車の前部座席で私の髪を元に戻してショットガンに乗っている

BACK INTO ENGLISH

In the front seat of his car is riding a shotgun with my hair back

INTO JAPANESE

彼の車の前部座席には、私の髪の毛を後ろにしてショットガンに乗っています

BACK INTO ENGLISH

In the front seat of his car, riding a shotgun with my hair back

INTO JAPANESE

彼の車の前部座席で、私の髪を後ろにしてショットガンに乗って

BACK INTO ENGLISH

In the front seat of his car, riding a shotgun with my hair back