Automobile Design

You should go with automobile design as your field of interest is to be a designer.

Check the below link for Automobile Engineering colleges in Kerala
https://www.shiksha.com/b-tech/colleges/b-tech-in-automobile-engineering-colleges-Kerala.

As you want to learn car modification, first you need to get an automobile degree in order to learn and then go for owning a car modification shop (custom car shop).

That is a very specific subject and I am not sure if a course exists for solely that topic. Automobile Engineering, Electronics Engineering or Instrumentation Engineering are some courses that deal with this topic.

You can apply, if you have completed a Bachelor's degree in any discipline of minimum 4-year duration from recognized university/ institute.

I will suggest you to skip a year and appear for the exam next year because it is difficult to get any good college with the above score.

It doesn't require Maths. It actually requires physics, which means Maths is a part of physics. So, it does require mathematics and without mathematics nothing can exists.

Automobile engineering requires mathematics, but automobile design doesn't requires maths. You can go for B. Des (Bachelor of design) in transportation and automobile.
1. NID
2. UID
3. Would
4. UPES
5. MITID
These top 5 colleges offers transport and automobile design. But, one thing you remember that, fee's regarding these colleges are very high.

You need to do some form of engineering or preferably automobile engineering. And then go to a foreign country study at a good University to get your master's and get a job. In India you can work at Tata or Mahindra provided you are from a very good University or based on your merit.