Select the polygons shown in red and extrude them inwards as shown. This will create a nice groove in the fender. Remember to delete the polygon at the center line.
Use the "Connect" tool to cut in the light blue edges. Chamfer the green edges by 0.2 or so. The finishes this part of the fender. Do any tweaking you deem necessary.
There is a chrome strip that fits in the groove in the fender and we are going to make that next. Once again, we are going to resort to box modeling because it's easier than polymodeling in this particular case. Create a box in front of the fender and extrude it along the groove as shown in the picture.
Split the geometry down the middle as shown by the green line then adjust vertices so that it fits better in the groove. That completes that part of the fender.
Create another box in the front viewport and position it as shown.