I did the same as Richard and flipped to LED, a nice blue one to match the Mustang main cluster - no issue with dimming but that may be just lucky
My car had LED clock illumination from new although the purple colour was ill matched to that of Mustang instrument illumination. Ironically it packed up giving me the opportunity to find an ice blue LED replacement. In typical MMC afterthought fashion clock illumination is wired direct to side light circuit rather than the rheostat/BCU control when any lights selected, hence dimming concerns not relevant. Chosing a low intensity LED replacement meant it was not overbright when other instruments are turned down for night driving.
