OK here's the problem. My old deck fried it's preamp RCA cables

How??

so
everyone was saying that I had a bad ground, (which I really doubt
since, I have it grounded to a metal bar in my dash,

Do you know that this metal bar is an adequate ground?

plus if it was a
bad ground I dont think the HU would even play,

It would.

I wouldnt think, also it
was making a loud screeching and feed back noise like on a bad PA
system.

Which is a typical symptom of a bad ground...

In the mean time while I was taking my deck around I installed
my old stock deck, Thats when I noticed my rear speakers may have had a
problem. It seems as if they would play when they wanted with no rhyme
or reason, but when they werent playing I would get aloud popping sound
form them, mostly when I go over bumps. well oneday I was washing my car
and had to go in the trunk a few times, I noticed whenever I would open
the trunk the speaker would starts working for a few mins then go back
out again say when I drove off and hit a bump or something.

Sounds to me like one of the speaker terminals may be grounding out
somewhere. Maybe on the trunk hinge? Take a look.

Well I got
my new deck a few days ago and installed it last night, I used brand new
RCA's to make sure my old ones were bogus, I Hooked everythng up and it
soundind fine I drove it around the black and the feed back noise came
back. I unhook the rear channel RCA's and continued my drive last night
and frove to work this morning (about 35 mins) and no problems at all,
so thet leads me to believe it is the rear speakers that are having a
problem. could they be blown or one of the coaxles have gone out or a
wiring problem or what?

Blown speakers won't whine. Check the installation. If nothing appears
wrong, you'll have to isolate components until you figure out what the
faulty device is.