If you're looking to remove the tint from your old car, it's definitely doable. You can either DIY or get a professional to do it for you. Just make sure you're following the proper steps and not damaging the windows in the process.
If you're looking for in-car sun protection, consider getting a baby car shade. It's super effective and easy to use. Just pop it up on your rear window, and voila! Best in-car sun protection, sorted!