I believe it is pressed in not much can be done, maybe drill it out... The ones for '69 are threaded in, but I don't think that applies for '70 up...
I came up with a slick redesign of the mirror that eliminates the cables so that the mirror is adjustable just like the passenger side. Nice for Mavericks that don't have the knob hole in their door. I have a completely refurbished set I'm about ready to sell. I agree that it is nearly impossible to replace the broken knob. I have one that someone successfully uncrimped the cables from but you would have to find a way to remove the cables from your broken knob.
OK, that really bites! I may drill it out order a new knob from npd, and add a dab of jb weld to the back its pressed it.
There's a trick in how to remove the knob. I think I saw this in a tech article on this site. The cables are color coded and it may take 3 hands to swap out the best I can remember
To straighten the cables I'd try lightly clamping a couple pair of Vice-Grips to the cable and bend it back straight... DONT clamp too tightly... Could maybe place bend between a couple blocks of wood and smack with hammer...
Someone here replaced the knob by drilling it out and replacing with the threaded one. I am in the same boat
This old thread will help you .............. http://www.maverick.to/mmb/threads/sports-mirror-inside-adjustment-knob-help.53899/#post-961647