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ままショットガンに乗っていた。

BACK INTO ENGLISH

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

INTO JAPANESE

私は彼の車の前部座席で髪を緩めながらショットガンに乗っていました。

BACK INTO ENGLISH

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