Seems like the switch that turns off the glove compartment light on these cars just BARELY does the job, and if the door is not adjusted tight, is warped or for some other reason does not push the switch button far enough, the light stays on. If this happens and you don't drive the car every day, you'll find the battery run down, maybe dead enough that the alarm system is activated and the radio is deactivated.
After getting some advice from various sources I made a tester fuse with a small light bulb across it instead of a fusible link and found that it lit up in circuit #15, interior lighting. Having someone in the car pushing the glove box switch button turned it off. We put a piece of rubber on the inside of the door with double sided tape to push the button further and the problem was solved.
So, if your 850 is inexplicably turning up with its battery down you might investigate this.