How to get the PA to work on old SEARS CB radio?

hey guys, i have a older SEARS CB radio that i bought at the flea market last year. it is a 5 watt? and has 20 channels. It has Two plugs on the back. one thats for an external speaker and one for a PA. the plugs are the same and my little homemade tester speaker that's a 3.5mm plug (standard for ipod, cd player etc) fits into the plugs and the unit recognizes it because it shuts off the internal speaker. when i speak into it i hear nothing, not even static. my question was do these CB radios have a special plug or does this plug just not work? ( i know my 3.5 mm homemade speaker works because i plug it into my ipod and can play music through it.)