OK, so I have a Cobra DXIII CB installed in my truck.

http://techshop.earthlink.net/p20805:S1151622

It has always seemed to work fine in CB mode.

I bought one of these PA horns to mount under the hood to use as a PA
speaker.

http://butleroutdoorstore.com/index....OD&ProdID=1433

Both the CB and the PA horn have the usual standard mini jacks on them.
Trouble is, when I try to use it, all I get is a horrible piercing
feedback whine when I press the mic.

At first I thought there was something wrong with the speaker, but when
I plug it into the external speaker jack of the CB, it works fine as an
external speaker. It even worked when I plugged my Ipod headphone jack
into it. So there can't be anything wrong with the speaker.

So then I assumed it was my CB that was broke. My dad has the identical
model CB, so I tried his out and it did the exact same thing. (terrible
whine in PA mode)

So either the Cobra DXIII doesn't work as a PA system, or I am missing
something simple.

I didn't have the speaker anywhere near the mic, so it's not like it
was picking up feedback through that.

Anyone ever hear of this happening?