If you’ve already “reset” the radio and checked settings like fade and balance, sorry, but you’re gonna have to break out a meter and check voltage levels either at the radio output or amplifier output or the speaker input to isolate the problem area. Whatever is easiest for you to get at would be my suggestion. You’re going to have to find out where your losing your signal. Super unlikely that all 4 speakers are defective at the same time. Pinched/shorted/grounded/open wires between the head unit and amplifier and/or between the amplifier and/or speakers could also cause your problem. Defective head unit and/or defective amplifier are also possibilities. Check out the schematics below in where to check appropriate wiring. If I remember correctly, the amplifier is mounted under the front passenger seat. It would be the easiest place to check your signals, both input and output. Back probing the connector terminals here would be the preferred method. Good luck!