Linksrheinischen Rundfunk was a private radio station in the Mainz area. We tuned into them while passing through the area in July 1988. I am not sure on what frequency we heard them, but I believe it was 103.6 MHz. According to a couple of websites, the station has gone through several name changes before being integrated into the RPR1 radio network. The transmitter was reportedly located at Kalmit. It was also one of the first FM private radio stations in Germany.
If you can confirm the frequency, or know any details about the station, such as its transmitter power and studio location, please contact me (philcobill (at) verizon.net.)