amp or sub wiring issue

I took the subs out of my firebird because im switching to stealth boxes and deicided to put them in my sentra. It has an old jvc head unit that doesnt have sub out plugs so i am useing the wires from the factory sub for the pre amp imput. Problem is the amp has power but the subs are getting little to nothing and when i turn up the stereo all the audio cuts off. I also noticed when i have the amp hooked up my other speakers are quiet but when i unplug the amp they go back to normal. I know this is very vauge but any ideas what i should look at. This is my first time trying to do this by myself and im kinda stumped

Since you don't have sub out out on the cd player you should be abke to go to any stereo place and get an adapter to use the sub out kinda like what you would have to do if you used a stock radio

Or buy a newer stero with sub outs.... lol

Yea im a broke college student fresh out of the army so a new head unit isnt really an option for now. I think i know what adapter you are talking about. Its the high imput or something like that correct?

I think but not 100% sure, but indo have the sony xplode with remote I took out of my z when I wrecked it ill toss to ya cheap if ya rather do that than buy an adapter

Pm me info on it

Will do this evening

I got a box thing from walmart when I put subs in my old truck. It taps into the speaker wires and a switched 12v source from the head unit wiring harness. It worked pretty good. It's blue and it was in the audio section hanging with all the adapter harnesses.