What is the right course of action?

Your answer is correct because according to German traffic rules, when no traffic signs or signals indicate otherwise at an intersection, the rule "right before left" applies. This means you must give priority to vehicles coming from your right.

In this situation, the rider on the bicycle is on your left, and the red truck is on your left too, so neither has priority over you. The blue truck shown in the rearview mirror is coming from your right, so you must yield to it. Since the blue truck is the only vehicle you must give way to, you are allowed to go first.

Remember this principle: At intersections without signs, you always allow vehicles approaching from the right to go first. This helps in determining the right of way clearly and safely.