The A3 and A4 share the same interface but the OEM install location is different. Therefore I would say you should get the A4 harness, even though you may choose to install the interface not at the OEM location below the passenger seat. How I see it, the plugs are the same but the length of the wires is different. On the other hand, if Lanny says Kufatec said take the A3 harness even though you are installing in an A4, go for it.