I figure on a 1991 its a direct spliced connection somewhere. There's no multiplexers or stuff like that. Probably not a relay. Look for crimp connections that are corroded or dirty.

Is the CHMSL in the window or on a spoiler? Maybe a broken wire after repeated flexing from 17 years of opening the trunk?

The 'controller' might be the little module that triggers a warning lamp if a bulb is burned out? I don't think that has any role in switching the lamps on & off.