Trapped Rydberg ions are a novel platform for quantum technologies, envisaged to combine the excellent control of trapped ions with the strong interactions found in neutral Rydberg atom systems. While in recent years trapped ions have been coherently excited to Rydberg states and were shown to be stable in the trapping field, strongly interacting trapped Rydberg ions have yet to be realized. Strong interactions between Rydberg ions are facilitated by microwave dressing. In this work the microwave dressing of a single trapped Rydberg ions is studied. The ion is used as a microwave field probe and the polarizability of Rydberg S- and P superposition states is investigated. This work presents an important stepping stone on the way to the goal of trapped Rydberg ion quantum gates.