If your car camping sleeping pad leaks air, you can try a few different things to fix it. First, check to see if the leak is coming from the valve. If so, you can try tightening the valve or replacing the valve with a new one.
If the leak is coming from the body of the sleeping pad, you can try patching it with a self-adhesive patch or a piece of tape. If the leak is coming from the seam of the sleeping pad, you can try sealing it with a seam sealer.