Kicker protection light won't shut off
There are 4 possible causes for the amp to go into protection mode. Kicker calls it SORT for Short, Overvoltage, Reverse polarity, and Thermal. If the amp has been in and working properly, it's probably not reverse polarity. And if it goes immediately into protection when power is connected, I'd rule out thermal. So it's probably either a short or overvoltage.
1) Start by removing the speaker wires and see if the amp powers up properly. If it does, one or more of the speakers is shorted or grounded.
2) Test each pair of wires with a multimeter set to the lowest resistance (ohms) scale. Measure across each pair and from each individual wire to ground. Across each pair, the meter should read about 3-4 ohms. From each individual wire to ground, it should read infinity. If the meter reads "0" ohms (shorted) in any test, you have located the problem.
3) If the speaker wires test OK, set the meter to read DC volts and measure the voltage at the amp positive terminal. If the voltage is higher than 16V or lower than 9V, you've located the problem.
4) If the voltage is OK, remove the RCA inputs and test for proper power up. If it powers up without going into protection, you have a shorted/grounded input. Replace them one at a time to determine which one(s) is/are defective and replace the cable.
If these tests show all the wiring is good and the voltage is OK, the amp is probably shorted internally (most likely an output MOSFET), and requires service at the component level. You need to take it to your local audio shop or send it to Kicker for repair.
Hope this helps.