I used to have a 1996 Civic VTi with the same radio.
Do you have the owner's manual? If I remember right, the manual says that if your HU gets locked due to entering wrong codes then you must bring it to the dealer so they can unlock it. However, I doubt if the Honda or Mitsubishi dealers can still service your radio but just try anyway. Fujitsu-Ten HUs used to be distributed by Diez Corporation but I think they have closed shop already. Maybe you can also try your luck in Banawe or Raon.