First off – I highly commend the effort to add wear and tear to the toy truck. You’re on the right track for sure. Unfortunately you may be approaching the limits of procedural grime/scratches, especially in terms of masking. The issue you describe of needing to exclude a certain area is tough to do procedurally (without painting custom UV texture maps)
Silent’s suggestion is valid about assigning a different material to the faces in the wheel well that you described. You could duplicate your shader, remove the scratches component from the duplicate, and assign to those specific faces.
