If I install a heated mirror on a Buick that was originally non-heated, will the mirror heat work?

you have to install the wires through a fuse and to a switch, but yes it will work. you installing the whole assembly, or glass itself?

You may also need a relay similar to a power window relay your fuse box may already be set up for the installation.

Where may I find instructions for this procedures

This answer might be a little out of date, but definitely relevant. You CAN do it another way, but I prefer not to hack wires. Find a car in a salvage yard with the heated mirror feature and take the wire harness out of the door. The features have always been in the car, but there is no wire going from the DCM (Door Control Module) to the device. Seat, and mirror memory are stored in the DCM, The Heated mirror, and heated mirror feature is also controlled by the DCM. For auto dimming side mirrors that is controlled by the rear view mirror, sending the signal to the DCM. No special fuses, or extra wiring is necessary. Just remove the wire harness you have, and install the harness that has that feature in your car, and you'll be good to go. I just did this in my car. I have a write up on how to do this and also how to install a tach display in a car that didn't come with one. I'll have the document links on the Lesabre page in a few days at oldmanmotors.tech Hope this helps! -John B-