losing left side of line level converter

I have a 1998 Jeep Grand cherokee that I used the stock stero wires BEFORE the factory amp to send sound to my amp. I bought a cheapo line level converter of of ebay and in about 3 months the left side went dead,sort of.It wouldn't have half the bass coming from that side even when I turned up the output allthe way. I just figured sincc it was cheap  it wasn't no big deal. But I went a got a maxxis from the local stereo shop  it did the same thing in about 6 months. I have all the wire right  as in the + to + and - to - so I can't figure out what the deal is?? It's only the left side, the right is fine.THe maxxis is a four channel line lever converter since that is all they had at the time and I didn't want to wait for the two  days  for a new one.(VERY SMALL TOWN). SO I figured I'd try the Front two channels since I was using the rear channels of the Maxxis but then it when I got it all hooked back up it was half the bass from BOTH speakers. IF I unpluged on the other would come to life and hit as hard as it used too. So I'm confused. I thought about using a 2-into-one RCA plug but not really sure that would be wise. I have a stock radio,stock Infinity amp, 2 12 MA audio DVD wired to 2 ohms (laugh if you will-they hit and sound pretty decent) and OK you can laugh and rollyour eyes now- a 1600 watt 2 channel ultra audio amp wired to 2 ohms.Hey, I have 5 girls. Think I get to buy myself the good stuff!!??

It sounds to me like you have the line out converter wired out of phase.  Reverse the wires on only one of the input channels to the Line Out Converter. 

Yeah I had one side out of phase but I know that I had it wired upright on the last converter. But I can't figure out why it keeps killing the left side?