To get to this trace, you must remove the radio's bottom cover. On each front corner you will see three Phillips head screws --- two in-line and a third to the inside. The "inside" screw threads into a long hex spacer. Remove this "inside" screw from each side. Remove the bottom panel.
The receiver mute trace starts from Pin 6 (count from the right!) of the only IC on the system board. This is a SIP-style IC, just to the right of center. From pin 6, it connects with a small glass diode, which goes to the left. The diode is soldered to the system board through the first via (H96). Find this trace on the bottom of the board. About 1/2" above that via you will see a second via (H95). Cut the trace between these two vias.
NOTE1: Confirm that you have the correct trace by checking continuity with the receiver mute pin --- Pin 7 on the receiver board. On the system board, right group of pins (P904), count from the left.
NOTE2: Do not cut the trace between the IC and the diode!
The receiver normally gets its RF from the T/R relay through a small grey coax jumper with RCA plugs on both ends. Remove this jumper and connect the new antenna lead to the RCA jack on the receiver (or preamp, if present).