I am on my second set of genuine mini spots and both sets have suffered condensation/misting. I don't actually think the dealer believes me so I have dealt with it myself.
1 Take the fronts off by undoing the single screw at the bottom.
2 Take out the bulb and then allow whole thing to dry indoors, you can wipe inside using a long screwdriver and kitchen roll.
3 Remove the chrome outer ring by levering out the 3 clips, be careful as they can fire off so cover with a cloth.
4 Run a bead of silicon sealant around the rim of the glass then re assemble. Use long nose pliers to replace the wire clips.
5 Put a further bead of silicon around the edge of the back before putting the completed fronts on.
Mine have now stayed condensation free ever since.