no sound from sub

This is the deal, a friend of mine asked me to install his system for him he has a 1993 ford taurus the speaker is a lanzar maxp154d, the amp is mtx ta4501 and the deck is an alpine cda 9847. first problem i ran into was when i connected the wire harness to the factory one no sound came out of the speakers on the inside the sub played fine. so then i disconnected all the inside speakers and hooked them up one at a time i had 3 hooked up and when i hooked up the last one all the others cut off and no sound came out the inside speakers. so i disconnected the last one the music came on, then i reversed the wires on the last speaker why i dont know and the music stayed on but it was real low coming from that speaker. I told him i didnt know why it was doing it he said it was cool. i just set the fader to just play the front speakers everything was good subs and all, bout 2 weeks later he came back and the sub wouldnt was making a little mumble noise on and off.  real low the sub would come on when he start the car in the morning then cut right off and go to the mumbling noise the amp is on and no protection light is on the sub is wired in parallel to a 2 ohm load the amp i think is like 500 rms the sub is like 1000 rms does anyone know what could be wrongIf the trunk dont rattle then its not loud.

If it's to loud then you're to old.

the radio cuts out because there is something wrong with the speaker or the wire itself. you can test this with a meter by setting it to continuity and hold one lead to any ground and the other lead to the suspect speaker wire if you get a reading from the meter then you have a shorted speaker wire, make sure you do this with the system off. next, test BOTH coils individually with an meter and it should read around 3-4 ohms for each one. if that checks out then try to shake the sub and listen for anything that rattles. if you hear it rattle then the voice coil is starting to get loose and your sub is toast. try to push the sub in and out and make sure it moves freely and it doesn't sound like the voice coil is rubbing inside the gap. if your sub checks out then you should inspect the speaker wire to make sure it isn't pinched or sliced open anywhere or melted. if that's ok, then that only leaves your amp to be the problem. check your signal connections and RCA wires to make sure they are in good shape with no cuts or tears in the jacket and that the jacks themselves are nice and snug in the connectors.