YOU SAID:

I was riding shot gun 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ほどいてショットガンに乗っていた

BACK INTO ENGLISH

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

INTO JAPANESE

私は髪をほどいてショットガンを持って彼の車の前部座席に座っていました

BACK INTO ENGLISH

I was sitting in the front seat of his car with my hair untied and my shotgun in hand.

INTO JAPANESE

私は髪をほどき、ショットガンを手に彼の車の前部座席に座っていました。

BACK INTO ENGLISH

I untied my hair and sat in the front seat of his car with a shotgun in hand.

INTO JAPANESE

私は髪を解き、散弾銃を手に彼の車の前部座席に座りました。

BACK INTO ENGLISH

I untied my hair and sat in the front seat of his car with a shotgun in hand.