YOU SAID:

I was at the car mechanic who will do the job with a car which got crashed

INTO JAPANESE

私は衝突した車で仕事をする車の整備士にいました

BACK INTO ENGLISH

I was in a car mechanic who worked in the crashed car.

INTO JAPANESE

私は衝突した車の中で働いていた車の整備士にいました。

BACK INTO ENGLISH

I was in a car mechanic who worked in the crashed car.