The question: OEM design using Balanced signal and processing or unbalanced twisted pair wiring? Balanced signals have a the same signal on two wires however one is inverted and the conductors are usually twisted together and shielded. Known as low impedance wiring used on stage rigs, recording studios etc. This arrangement can send signals 800+ feet with little to no artifacts. Rockford, Soundstream, Zapco offered balance systems to increase voltage and reduced noise. The downside was using specialized cables to connect. Wiring is one Hot, one Cold, one shield.
Unbalanced, high impedance wiring uses a single conductor and some type of shield. This type is good for 20feet or so. Subject to interference, ground loops, signal loss. Standard RCA cables. I have seen a few companies using a quasi "balanced" designs using RCA cables.
The OEM hybrid: Unshielded twisted pairs. Used in telephone backbone systems to send signals over longer distances and provided noise rejection without shielding. One wiring will be the "shield" the other is the hot signal.
True Balanced line converted to unbalanced, combine the shield and the Cold wires as the shield and connect the Hot as your signal conductor. Without combining, the signal level will be low.
Many newer vehicles are using integrated systems. My caution here is know what you are cutting/tapping into!!!! Audio/visual signals are now integrated with control and other signals, CAN bus is one example. CAN bus uses unshielded twisted pair, similar to above. The electronics mod your HU is connected to might also turn on the wipers, seat heaters etc. The connectors might look familiar but it doesn't mean you know what language is being spoken.
An example: The Ford Sync system sends a fixed volume signal from the HU to the factory amp/processor. Volume is controlled via control signal to the processor . This means for integration you don't want to tap signal from the HU. This leaves the speaker outputs, the best option for LOC. PAC does offer a convertor but has some issues. Do research before diving into newer systems!