For anyone interested: Use contract ID: THPPS30 .
It's the contract ID from a website called "germanwho.com" (you can also sign up for free there, you'll get the same discount).

Apparently it gives up to 20% discount, yet for my current booking, I'm getting 30% (!). It saved me a ton of money, hope it helps out others.

In return: Does anyone have a good "coupon code" I can use for a 2 week rental in Vegas next month?