Post does not say what model year we are dealing with.
Regardless, your problem has to be in the ignition switch. There are multiple gangs and one is still making connection even though the switch is off.
I have read other posts about similar problems. Some ppl report it's in the switch. Others report it is in the linkage inside the column. In this case, the recommendation was to get a new column from the salvage yard.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.