That's the next big question, my guess is that he was hired by Park City somewhere between 1986-1991 when the last killings/communications stopped.
From an April 10, 1996 Googled transcript:
2.
RESOLUTION APPOINTING DENNIS RADER (COMMISSIONER
GWIN'S APPOINTMENT) TO THE SEDGWICK COUNTY ANIMAL
CONTROL ADVISORY BOARD.
Mr. Plummer said, "Next you have a Resolution appointing Dennis Rader to the Sedgwick County Animal Control Advisory Board. That's Commissioner Gwin's appointment. I recommend you adopt that Resolution and make that appointment."
MOTION
Commissioner Gwin moved to adopt the Resolution.
Commissioner Hancock seconded the Motion.