Remember that you are also interviewing them. They won't expect you to know all the answers, but will want someone that they can work with. If you can, answer questions with the STAR method (situation, task, approach, result), but don't waffle. You can use one piece of experience in a variety of ways: teamwork, research, urgent deadline etc.
It's ok to say that you are nervous, they should try to put you at ease.
You may be asked 'trick questions', these are not usually to to you up but to see how you work an unknown problem. There is no right answer. Not knowing stuff is ok. Not being able to think up a plan is less so.
Remember whatever the outcome, this is really useful experience. See if you can get a site tour, ask about the tech used... You can then add this to your knowledge for later. In my experience, industry is frequently several years ahead of academia so you get a good chance to understand the real world.
Not sure I understand the problem fully, but you want a circuit to operate when you push the horn button, without affecting the horn operation.
Chances are the relay coil is drawing to much power.
Have you thought about adding a transistor to your circuit? It would draw very little current from the horn circuit but should allow you to drive something else. - such as your relay. It would of course require you to do some electronics.