[align=left]I have a 89 5.0 mustang.. and all i have on it so far is magnaflow exhaust.. but i took out the old radio and i got a harness and everything, but the radio plays horrible because i believe, of the amp thats already in there behind the radio.. any help on what to do?
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also, the radio i bought is a sony xplod and its got a built in amp in it already..

 

was there a octagonally shaped metal box with a bunch of holes in it under the deck? That is your amp. There are two hrns's that go into it. One will look EXACTLY like the hrns I am pointing to on page 2 of my cardomain page in my sig. If you turn the deck on, and disconnect that hrns, the sound should quit. Your hrns will be under/behind the glove box. There should be some tape holding the hrns in place, cut it and then pull the hrns up to the dash. If you use a ford 10-1770 hrns and wire it up red to red, yellow to yellow, ect and plug it into the hrns you pulled up there you will bypass the amp, and get better sound.