I just did that. If you have an intercom, all the required wiring runs to it. Get a 4pdt switch from digikey or mouser electronics. You'll see 12 contacts in three rows. Find the wires that run to the intercom's ptt input, mic input and audio input. put those wires and the common ground on each of the center row terminals of the switch. Run one side (row) , in the same order, of the contacts to the original radio, the second to the number 2. If you don't have an intercom, use the wires that run to the headphone and microphone panel jacks instead of those to the intercom.
good luck,
Marc