You arent giving us enough information to help you out.. In my experience "multiplex wiring" is the vehicles CAN system. Usually a CAN-HI and CAN-LO that are compromised of two twisted pairs of cables that run throughout the vehicle. I dont see how this would matter in re-installing your old radio unless you really hacked up your wires. Make sure also that you didnt cross your constant 12V supply wire and your accessory wire, since that would make the radio stay on constantly.