This is a trick I used on another car. Once you get the cap off, drill through the plastic cap and through the first metal portiion of the cap. BE CAREFUL TO NOT DRILL ALL THE WAY THROUGH BOTH INNER METAL CAPS and use a small drill. Use a sheet metal screw to fasten the outer plastic cap to the inner metal part. You will have defeated the "ratchet" function and the cap can be removed and replaced normally.
If you simply can't get the present cap off, do the above in place. By pinning the outer and inner together, you will be able to remove it.