I hope you got this by now, but for others researching in the forum...
First step sounds goofy, do your turn signals make noise?
6J3 option has the wiring setup to run the cop radios through the stereo speakers. When I bought mine they left the connectors under the dash unplugged when they pulled the cop stuff. Just plugged them in and good to go. Also got my turn signal noise back. Not sure when that started going through the speakers, I'm so old it used to part of the little relay can that you replaced.
FYI, same deal with the horn. It was connected to the siren, they pulled the brown horn wire and did not plug it back in when they sold the vehicle.
Strange having one side work, but I would still check that plug under the dash, downstream of the radio connectors, to see if they got creative in there.
