Reliable cellular connectivity is crucial for connected and automated vehicles; however, evaluating how communication links behave under controlled degradation during real-world driving remains challenging. This work introduces a Connectivity Controller, a hardware-software co-design platform that enables deterministic, scenario-synchronized manipulation of connectivity conditions in on-road experiments. The controller integrates advanced robotic test automation and a network emulation engine within a dedicated modem, and exposes an API that allows external systems, including ATOS, an open-source scenario execution framework, to trigger impairments at precise spatial or temporal points during a driving scenario. To validate the approach, field-track experiments are conducted using a dual-modem setup in which one modem is subjected to controlled RF attenuation while the other serves as a baseline reference. The evaluation focuses on RF-layer behavior demonstrating consistent attenuation, repeatable power alignment, and isolation between impaired and non-impaired communication paths under real vehicle conditions. These results indicate that the proposed approach can support controlled and repeatable connectivity experimentation in realistic environments. Overall, the Connectivity Controller provides a practical platform for studying vehicular connectivity under controlled degradation.