When your vehicle is defective, it causes an accident, and you're injured, you may be able to sue based on those defects. You're likely facing medical bills and other costs, and you may deserve compensation.
One important thing to note is that, while working on your vehicle doesn't necessarily invalidate your claim, significant modifications could do so. One of the three criteria that needs to be met for a strict liability case is that you have not substantially altered the vehicle's condition.
A substantial change is thought to be one that impacts performance, not just cosmetics. Changing the paint color doesn't make a difference, but altering the integral systems does.
For instance, perhaps your vehicle is an SUV that is somewhat designed for off-road excursions. You want to make it even more worthy on the trails, so you add a lift kit. This isn't a factory job, it's something that you do in your garage or that you pay someone else to do for you. It gives the SUV an extra four inches of ground clearance when you're on the trails.
You're free to alter your vehicle if you so choose, but just know that any future issues with the systems you altered may not be covered. If something breaks on the highway and causes an accident, the manufacturer may claim that your alterations were the problem, not the original job they did when building your vehicle.
Many people don't think of this until it's too late. Be sure you know all of your legal rights in advance so that you understand how to proceed after an accident.
Source: FindLaw, "Defective Motor Vehicle Lawsuits," accessed Aug. 03, 2017