I added a RCA mono input by soldering a wire from the RCA jack to the AM source wire on the switch. I can listen to the input by tuning away from a station and switching to AM. (I could have installed a switch to break the AM wire.) I soldered a wire to the tab (circled in orange) where the AM preamp yellow wire is soldered (picture before soldering). The outer part of the RCA goes to the case ground. I grounded my jack by attaching it through one of the holes in the rear. John
