A transformer would work but you have to use a good one or you might strangle your low bass response. But before getting into that, there is something about your problem that doesn't sound right. That your radio head line outputs do not share a common ground implies that they are not line outputs at all but bridge-type speaker outputs. Are you sure those are line outputs? All of the line connections that I've seen in two part car stereos use RCA connectors mounted on the chassis, generally using the chassis as ground. If your head outputs are just wires or just come through a large plastic block type connector, that also makes me think they are speaker outputs.