Electrical problems with passenger side downstream o2 sensor

First time posting on here so bare with me,
Bought my 2006 4.6l 3v v8 mustang about four months ago. Had catless xpipe which I switched to the stock exhaust. Couldn’t get my o2 sensors to ready so I kept driving it and still nothing. I then replaced one of the upstream and one of the downstream sensors. All of my o2 sensors started working except the passenger side downstream sensor. I replaced the clip that attaches the o2 sensor to the wiring harness and still no voltage reading for the passenger downstream sensor. I had a friend test the voltage of all four wires going to that sensor and he said only one of the wires has a bad connection/voltage. So that led me to checking the grounds by the hood prop and still no luck. My guess is either a secret tune on the car or a bad wire somewhere over the transmission. Any advice? Ideas? Any advice for how to check the wires that go over the trans without dropping it or bringing it to a shop?