YOU SAID:

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

INTO JAPANESE

私は彼の車の前部座席で髪を解いた状態でショットガンに乗っていました

BACK INTO ENGLISH

I was riding a shotgun with my hair untied in the front seat of his car

INTO JAPANESE

私は彼の車の前部座席で髪の毛をほどいてショットガンに乗っていました

BACK INTO ENGLISH

I was riding a shotgun with my hair untied in the front seat of his car