OT Audio Adapter Query

[looppool@cruzio.com]

I'm having an audio adapter dilemma and wondered if someone can see what
I'm doing wrong.

I just purchased a really cool Dayglo Orange puffy backpack that has
a small 3 watt amplifier in it with two stereo speakers built into the
backpack.

This backpack has a hard wired stereo male mini 1/8" plug that is supposed
to hook up into an Mp3 or portable player (taking the Mp3's output to be
amplified).    It works in this capacity.

I also have a strange instrument from the 60's called a 'Sonica'
that is , essentially, a ribbon controlled touch organ.

It has built in speakers but a Mono speaker out (that disengages the
built in speakers when plugged in) that takes a mono male mini 1/8" plug
(sending the signal out to an external amplifier).

I went to Radio Shack and purchased a stereo female mini 1/8" (to plug the
backpack
amp into)  that goes to a male 1/8" mini plug that plugs into the audio
out of the Sonica.

I can't get the Sonica signal to amplify through the backback.

I've tried all components out with their respective appropriate connections
(Sonica to amplifier,  Mp3 player to Backpack and the plug works

What am I getting wrong.
I'm stumped.