Hello Elfman.
I'm betting that your cabinet speaker amplifier chip has developed an internal short from Pin 5 (+12V) to pin 4 (output pin). This would explain why you see 12V on the output. There is a chance that the output is not shorted and the amp chip is railed to +12V due to another defect inside the chip. The amp chip is inexpensive ~$3 so just replace it if you are capable to doing the work.
I hope that helps.
Mark