After diploma, you can go with degree level courses or PG diploma programs. B.Tech degree can be obtained with specialization in Automobile Engineering or Automobile design Engineering. Admission to the course can be done through lateral entry and thereby you can save one year.
B.Tech degree will help you to get more good jobs with higher salary.
A Post Graduate Diploma in Automobile Engineering is also suitable for you. Or else you can try for a job with railways, defense, government departments etc.