What causes Ford Airbag Code 18?

Hi. I have a 2005 Ford F150 with the 5.4L Triton engine. I had a radio installed from a local shop. Now the Airbag light blinks code 18 and the penger airbag disabled light does not come on. I checked airbag fuses 19 and 27. The fuses are fine. What are the likely causes of this code? Thanks!