subwoofer and amp dont work after hook up

i have a JVC headunit and i wanted to add a small subwoofer in the trunk of my 240sx just cause the rear speakesr are out and i want a little more bass i got all the wiring part done since the previous owner had a system in it, the power and remote wire were already there, just a matter of hooking them up, i mounted a new ground wire and i hooked it all up to the amp now the amp was also from the previous owner, and i have no clue what type it is or whatever, it was just there, he siad he was using it to power his speakers and never ran a sub, not sure if that matters, it also says 120w x2 on the cover the sub has all of these connections -power -ground -remote -1 white RCA -1 black RCA -left speaker - -left speaker + -right speaker - -right speaker + and it also has these settings -subwoofer level: 80, 120, "subwoofer off" -mode: st, brg so i was thinking today, if i just bought a cheap subwoofer and hooked it up, i'll have some bass right? so i got one from work, cheap 15 dollar POS that was on clearance, and tried to hook it up i hooked up the power, ground, remote accordingly, and also the RCA cables, the power cable near the battery terminal didn't have a fuse in it, so i put in a 25amp fuse, the amplifier itself has a 30amp fuse in it, and it's still good then i took regular speaker wire and i connected the subwoofer + to the "left speaker +" and the subwoofer - to the "left speaker -" then i turned on the radio to see what was up, the amp turns on, and the subwoofer turns on about 1 second later, and i hear the subwoofer bump once, then the rest i hear is slight low volume thumps, it is always in the same pattern, bump..bump..bump..bump.. and it just keeps going when i play music, then if i turn off the radio, the subwoofer turns off and i hear a small thump before it goes off, also the subwoofer doesn't bump to the music at all i got a couple questions -did i hook up the subwoofer correctly? i'm only going to be running one subwoofer, it is rated at 200w -what should i set the amplifier to? ST or BRG? -what subwoofer level should i set it at? 80 or 120? -would a different amplifier with more power solve my problem? -the protection light comes on with the amp, i'm not sure what that means thanks a lot to those who answer my questions, i tried to be as in depth as i could