I can't speak for a Mark LT. However, the five digit code for both my 1989 Mark VII and my father's 1997 Town Car are located in two places, attached to the passenger side trunk hinge on the inside of the trunk and on the outside of the control unit for the keyless entry systems, which are mounted under the package shelves in both vehicles. Wherever your code sticker is attached, it would be in a spot accessible only with a key, but easy to reveal or see once you are there. You shouldn't have to remove any carpet or trim panels to find the code label. The control unit itself, with a code label attached, is most likely behind a trim panel or cover, either within the interior or under the hood.
If all else fails, just call the service department and ask them over the telephone where to find the code label. I must also add, that given that there are only five buttons (two consecutive numbers per button) there are only 120 5-button code combinations possible. If push comes to shove and you have the patience, you could stumble on your code within two hours of trying consecutive 5-button combinations.