Kicker amo in my old vette
If the amp is in protect mode, it probably has shorted output transistors. To eliminate other possible problems, do the following.
With your multimeter set to DC volts, the black meter lead on the ground terminal of the amp (not on the point where the ground wire connected to the vehicle) and the head unit on (so the amp will have remote voltage applied), touch the red lead alternately to the B+ and remote terminals of the amp. If the voltage drops below ~11 volts, you need to check the wiring feeding whichever line is dropping too low when the amp goes into protect.
If the voltage is OK, you need to check the speaker wiring and the speaker(s). Try it with a known good speaker if possible to see if the amp works normally.