the bit the mirror connects to is broken mate so you need a complete unit

simple enough to unscrew from the other mirror but it's the wiring that's going to be the tricky bit as i can't remember if it was push or soldered to the sierra units, the later mirrors are all push fit clips that are easy to strip down

you can also clearly see the little prongs that hold the cover to the case in the right of the picture, top and bottom, with their little rubber bung things to spread them open and also on the left upper in the last picture, just above the wiring loom

easiest thing to do is to strip down the mirror with the 4 screws out and see how it's wired up, removing the rear cover gives you virtually nor access as it's just a cover over the top