You'll need to have a helper at this point, as it's necessary to keep the work-piece and the router bit lubricated at all times during cutting to ensure a clean cut and that the aluminum doesn't fuse onto the steel bit (galling). Have someone spray a constant stream of WD-40 or dry silicone lubricant directly onto the bit and work-piece at the point of contact to keep the cut going smoothly. Though a bit messy, it's the only way to cut the piece properly. The silicone lubricant is nice because it evaporates without leaving behind an oily residue. I should really be wearing gloves in this photo.
After sanding and polishing the chamfer, the piece now looks like this. Now the final finish can be added to the aluminum surface. This can be done with 180-grit paper on a D/A orbital sander for a random swirled surface, or it can be done with 180-grit paper on a sanding block for a brushed finish. The choice of surfacing is all personal preference. This piece will be brushed to match the factory trim in the vehicle.