All threads are right handed (clockwise to tighten) unless there's a rotating part that will tend to unscrew the thread during normal use - then they use a left handed thread - very rare - your wing mirrors will both be right handed thread form.
Screw mirror up tightly and work out how to adjust it to the correct position without snapping it - an adjustment at the other end probably.
Good luck...
Peccavi
November 2009