I contacted them via email (
reservations-usa@sixt.com - maybe the wrong email?) and attached my Avis card PLUS an email newsletter with my status. They sent a link to their "current partner list as you may qualify for a status match:
www.sixt.com/sixt-partners/". Huh? I replied. They replied saying they needed proof. Huh? I sent a third email and attached my card and newsletter proof again. THEN I got an email approving me for Platinum.