In automotive software engineering, system-level regression testing is crucial to ensure proper integration of oftentimes safety-critical components. Due to the inherent complexity of such systems and components, testing is commonly performed manually and in a black-box manner, which is particularly costly and leads to slow feedback cycles between testers and developers. Regression Test Prioritization (RTP) aims to reduce feedback time by ordering tests to reveal faults earlier during the testing process. However, most prior RTP research does not incorporate varying fault severity, which must be taken into account when evaluating and designing appropriate RTP approaches for safety-critical automotive software systems. In this work, we present a case study at our industry partner MAN, a leading international provider of commercial vehicles. We design and instantiate a domain-specific, severity-aware RTP assessment model and comparatively assess state-of-the-art RTP approaches. Our results indicate that simple and partly well-known heuristics based on test history and test costs have the best cost-effectiveness, achieving between 85% and 90% of the maximum possible feedback time reduction. On the other hand, search-based and machine-learning-based RTP approaches do not perform better, especially if available test history is sparse.