Spoiler alert. Its very easy and super cheap! Your 2008 and up police interceptor already has the hardware necessary (drivers door module). All you have to do is enable it with Forcan (a free program). Below I will list the products and resources you will need (Paid Links).
Forscan Download Link https://forscan.org/download.html
Forscan Extended License Link (Must create an account) https://forscan.org/forum/extlic.php
USB OBDII Ford Programming Adapter https://amzn.to/37b80br
Ford Keyfob(s) https://amzn.to/2W8T3Qw
Binary To Hex Converter https://www.binaryhexconverter.com/binary-to-hex-converter
Hex to Binary Converter https://www.binaryhexconverter.com/hex-to-binary-converter