Sad In't it.
On the upside, you shouldn't need any tools to replace mirrors......for a long while I carried the right sized allen key in me pocket cos I have a tendancy for chucking my adv down tracks and dropping it in ditches......but it's dead easy to undo (revolve each one the way it would go if/when it goes when hit, ie clockwise on the RHS and counterclockwise on the LHS) (ops, should check this, not knocked off the LH mirror yet)......
Rather than digging around in a tool kit to replace or tighten, just shove a digit up the hole underneath to trap the allen bolt a little, then spin the mirror stalk around until it tightens enough to be stable...if it ends up facing the wrong way, loosen again, allow allen bolt to spin a little then repeat until position is satisfactory.
Easy peasy lemon squeezy.