UK3 : ELECTRONIC SYSTEM, ACCESSORY CONT

Done the research, and cannot figure out exactly what it is. I was reading the '88 service manual, and it's in coordination with the performance sound system on the firebirds. There are variations of the wiring schematic that has to do with with or without UK3, and with and without subwoofer. I thought maybe it was the dash switch, but both systems have it, and then i thought it was steering wheel controls, but it seem that the diagrams have that as well if my memory serves me right. What exactly was UK3.

Also curious, would steering wheel radio controls that fit into an '88 steering wheel by any chance work the radio controls for a 1999 Firebird radio? Doubtful that the resistances are correct, but figured maybe someone can give me an exact answer as i haven't found the resistances of the '88 model year yet, just the '99. Thanks!