my sub doesnt work when the car is off

i have this friend of mine that has a 2005 scion adn the factory stereo is still  present and he has wired an amp to the factory stereo . the question is when the car is in accesory mode (off) the subwoofer doesnt work it hits once then fades lower till you cant hear it at all any suggestions on whats wrong with it1993 sdv

Alpine 4x6 6x9 5 1/4 swr-10d4

alpine v60

12.5 VGA flip down

Nettop pc w/10" touch screen

78nova

5-re audio 5x7 ,10" sub &

Xtx-500.5

"Take the faceplate off get the jumperpack"

What is the system voltage when this happens?Support the12volt.com

I never put it on the voltometer but I asked him on his wiring and it all seems normal except his remote turn on. He didn't wire it to the anttenna he just found a wire that is hot when the ignition is on. So I don't kno if that can be the issue1993 sdv

Alpine 4x6 6x9 5 1/4 swr-10d4

alpine v60

12.5 VGA flip down

Nettop pc w/10" touch screen

78nova

5-re audio 5x7 ,10" sub &

Xtx-500.5

"Take the faceplate off get the jumperpack"

Yes, that can be the issue since many vehicles these days do not use a full 12V for trigger voltages.  Check the voltage of whatever is being used for remote turn on.  But, if the amp comes on for a second and then fades out it sounds to me more like crappy main voltage.  If you want to run the system with the car off, you'll probably need a new battery.Support the12volt.com

Thanks for yoyur help ill double check the remote turn on and the main 12v1993 sdv

Alpine 4x6 6x9 5 1/4 swr-10d4

alpine v60

12.5 VGA flip down

Nettop pc w/10" touch screen

78nova

5-re audio 5x7 ,10" sub &

Xtx-500.5

"Take the faceplate off get the jumperpack"