What is correct in this situation?

You correctly identified that the "right-before-left" rule applies here. This is because the triangular sign with a black cross inside a red border indicates an intersection where the general rule "right-before-left" is valid. This means you must give way to vehicles approaching from your right, unless other traffic signs or signals indicate otherwise.

In this image, no other signs or signals override the rule, so you must yield to the vehicle seen on your right side and the one in the rearview mirror. This right-of-way rule is fundamental in German traffic when no priority road is marked.

Understanding this rule helps you safely navigate intersections without priority signs by always watching and giving way to vehicles coming from your right.