If you don't read anything near a short from pin 5 to ground, look at the attached datasheet, ages 5 and 6 to see if you can see any problems with the input to the IC.
Yes that's no power. Off the top of your head, was this a common IC in the late 80's early 90's? It's definitely discontinued now, I'm wondering about harvesting one from another old amp.
I don't know if anyone other than Pioneer used it in car amps. The simpler UPC1237 was more common but generally only used in Japanese amps.
Pioneer still use their version of this IC, the PA2027, as far as I know.
Pioneer still use their version of this IC, the PA2027, as far as I know.