Audio Systems: Cant Ground, chevy blazer, model rpc

Question
Thanks for your answer.  Yes I do have wiring adapters (there are two for this vehicle), but the ground is separate from these.  All of the wires matched up between the harness and the radio, and I connected the ground wires (radio and van), so I thought it was an easy changeout.  But when I turned it on I got very poor sound from only one speaker.  I thought it was the speakers so I changed those wires around but there was no difference.  I hooked the factory unit back up and got the same poor sound from one speaker until I touched the black/green ground wire from the van to the unit housing.  Immediately all four speakers kicked in.  That's why I think it is a grouding problem.  I tried running a wire to the (-)battery post and got no change, not even a spark.  The radio has power, but it sounds like it has no amplifier to support all four speakers.  If there's anything else I can try, please let me know.  Thanks again.

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Followup To
Question -
I took a Blaupunkt Model RPC 430 cassette unit out of my 84 Chevy Blazer to put it into my 94 Dodge Ram 150 Van.  The head unit has a ground wire, but the factory unit was grounded through the unit body by a black and green wire.  I tried connecting the Blaupunkt ground wire to the black/green in the van but nothing.  I tried wiring the Blaupunkt ground to the chasis but nothing.  Just to make sure it wasn't a problem with the head unit I put it back in the Blazer and it worked fine.  I don't know what else to do.
Answer -
Jason,

 ok are you using a wire harness adapter ? if you cut the factory plug off if you can wire it back on and pick up a harness adapter and wire it in just so you can make sure the power/turn ect.. are all connected .. if you cant wire it on test the 12volt constant wire and the 12volt ignition/accessory wire to make sure the constant has juice all the time and the ignition gets juice when the key its on.. if both of these work try runing a ground from the battery and wiring it in to see if it makes a differnce if it does use a test light with the ground lead on a hot wire and probe for a good ground then drill and connect it to your unit  .. other than that i dont know what to tell you besides running all new wires but this should get you a ground and get it running .. hope this helps you out if you need anything else let meknow

thanks  

Answer
Jason ,

 this is a head scratcher =\ .. have you tried grounding the case of the cd player ? like you did the factory unit ... give that a try and see what results you get ... you have already tried everything that comes to mind from me at this point if i was there to look at it maybe i could get a few more ideas but on the adapters you purchased if they are schroshe they have a tech line give them a rind maybe there is something that is vehicle specific to the van that they might have on record other than that i'm at a loss you might try asking a dodge expert on here in auto repair maybe they can give a better lookinto this than i can .. sorry

thanks

Cody