WARNING: This page is about a different car, the 2006 Mercury Grand Marquis and 2006 Ford Crown Victoria. However, it is still accessible from the selected car via links, so may be relevant.
The following lamp fault code (LFC) indicates the specific fault associated with diagnostic trouble code (DTC) B1891:
LFC DESCRIPTION
LFC
Description
10-15
Air Bag Tone Warning Indicator Circuit Short to Battery