That basically amounts to a source selector. Refer to our application note "Dealing with legacy pin 1 problems" for information of how to wire an RCA connector to the UcD module. Install a DPDT switch to connect the hot/cold inputs of the UcD module to either pins 2/3 of the XLR connector or to the signal and shell contacts of the RCA connector. Note that pin 1 of the XLR connector is not switched, but remains hardwired to chassis. The RCA shell contact also remains hardwired to the chassis. This means that when the DPDT switch is set to RCA, the cold input of the UcD module is grounded via the RCA connector. See schematic in this the application note "Dealing with legacy pin 1 problems"