- Apply scale.
- Clear existing seams
- U>Smart UV Project
That’s a quick and dirty way to get something working quickly.
When you have more time, look at some reference images of car seats and their upholstery. Where you see stitching, that’s where you want to place your seams for UV unwrapping. Clothes and furniture (including things like car seats) are some of the easiest things to UV unwrap correctly. They are literally made from the nearest real world equivalent… a pattern drawn onto something flat which is cut out and stitched together.
I’m not sure what is going on exactly with your model/file. Did you make the seats yourself? Where is the normal map actually from?
All sorts of things seem to be exacerbating the issue.
To fix it, I’d separate out the parts of the seat which are hard-edged (like the hardware) and then delete the custom split normals on the seat, turn off or adjust autosmooth on the seat. Try a different normal map you know is good, making sure to set it to non-color instead of sRGB.