I am looking into doing this to my car as well, I work at a ford dealer in the parts dept, so tomorrow i'll look at our catalog and see if they show different wiring harness for cars with and without heated/turn signals mirrors. I'm almost willing to bet that they didn't use multiple harness for these cars so I bet the cars are already wired for and it would be plug and play. Another way to check is to pull your mirror off and see how many wires are going on the pigtail going to the mirror and then see how many connectors there are in the socket it plugs into. Example: pigtail may have 6 wires and socket may have only 3. so there for the other 3 wires can be your heated and turn signals.
I know a guy at work had a bought a 12 F150 and didn't have heated nor signals, and he bought OEM trailer mirrors with heat/signals. And he plugged them in and they work with no additional wiring