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?