Just chiming in on this one, I believe the RadioShack mini amp needs a powered mic in order to operate. I noticed the one Batninja linked to requires a button cell battery, so I assume that powers the mic itself.
(Just from my own opinion, I have owned this model and I'm not a big fan of "around the back" mics. They always seem to stick out too far in back and interfere with my helmet placement. This one claims to have an "over the head" strap, but it's too short to be used for me. Then again, maybe I'm the freak who's giant head doesn't fit standard headsets...)
