Get into the wiring. I just finished mine this past year on an 04 FXT. I bought the iWire plugs to finish it completely with the auto harness but didn't end up using them because I was able to source a manual harness, ECU, ABS module etc. I still have the plugs/wiring if you're interested in it at all. You can reuse/rewire some of your auto harness to make it work as well. Run through this thread again, it'll likely be more clear when you get into reading along and looking at the actual condition on the car.
I do know, switching to the full manual harness, I also needed to swap the ABS module. I wouldn't expect that to be an issue if you're still using the auto harness, and as someone else pointed out, splicing out one specific wire.
That said, I was also able to get mine to work with the auto harness, temp wiring and test drive it before swapping to manual. Somewhere along the way I lost my cruise control. It activates from the dash, but doesn't work otherwise.