WRN is an aggregated output from the BEP 3.0, meaning that it combines several possible warning sensors into one indicator using diodes. This means that if any one or more of the sensors triggers, the indicator will light up. Then you have to figure out which sensor or sensors tripped and fix it/them.
The sensors WRN aggregates are brake lamp test (BMU), ABS, Alternator charge light, Water temperature, Oil pressure, and Choke. So, the question is whether you've gone through to diagnose all of these sensors (except ABS since you have an 1985 K100).