I don't know about cards like that, however people put stickers of one of the police unions (GDP) on their windshields in hope of getting a better treatment from the police.
However, there are two reasons why this would not work:
1) Police officers can choose to forgive minor infractions, i think everything below a certain fine can be waved. An example would be if you drove a little to risky or something else and don't behave like an idiot to the officer.
2) as soon as you are above these thresholds, it gets very dangerous for cops to let you off the hook. There is a crime called "Strafvereitelung im Amt", basically meaning trying to hide a crime as an officer of the law. it carries a sentence of at least 6 months up to five years and even trying to hide something is punishable. This can also break the career of an officer. So if you, for example, drive drunk and your buddy let's you off the hook, he should damn well be sure that his partner does not talk about it.
There is, of course special treatment for special persons. I remember the mayor of my town, being known as an alcoholic, driving his car at night away from the bar where he boozed it up with his buddies. He never got caught, probably because his license plate was known and the cops were "somewhere else".