line out convertor

I have a line out convertor to install. The RCA's are needed to power an amp running an enclosure and I am using my factory headunit. I was going to install the LOC on just the rear speakers b/c someone mentioned to me that if I wanted I could adjust the fade to the front and it would lower the bass, which is pretty neat. well there's problems.

1. My LOC has 6 wires. 2 ground(+/-??), 2 left channel (+/-) and 2 right channel (+/-). Do I just run the 2 grounds to the body or do I need to splice em in somewhere?

2. My rear speakers do not have any bass going to them. All the bass is in the fronts. It is a factory  Infinity system in my 02 dakota club cab. It has a factory amp as well. Do you think the bass is being filtered at the amp or directly out of the headunit. If it is not being filtered until the amp then I should be OK splicing into the wire harness correct. And if the factory headunit is filtering the bass then I need to splice into the front speakers, correct?

Thanks for reading this, I truly need some help here, this install is on the 3rd week and I really just want to get my truck back together. ANY help would be GREATLY appreciated.

Thanks again, Steve

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1984 Buick Grand National
2002 Dodge Dakota 5.9 R/T
1987 Oldsmobile Cutlass