For Mustangs with the standard mirror, might fit other Fords as well. $90+shipping new. Your price $50 shipped USA. This one is for Escort radar detectors (ie 9500ix).
https://store.blendmount.com/blendmount-radar-detector-mount-for-beltronics--escort-ford-mirror-id-for-001-p1796.aspx
https://store.blendmount.com/blendmount-radar-detector-mount-for-beltronics--escort-ford-mirror-id-for-001-p1796.aspx