external onstar speaker

So I have an 07 NBS Chevy Crew Cab with out BOSE.  I installed a Pioneer AVIC D3 with a Peripheral GMAHLAN and PESWIPS.  Everything works great......I have a small handsfree speaker from a Cell car kit as my external speaker for Onstar.  The volume on the GMAHLAN is all the way up, but I need more volume.  Any suggestions of powered external speakers?  And one thought is I don't know the OHM rating on the handsfree speaker, maybe that effects the level as well.

TIA  Ned

Heres what I ended up doing....FYI

The hands free speakerfrom the cell kit  was too quiet and sounded tinny, so I tried a Uniden CD external speaker, louder, but too big and sounded just ok, so Radio Shack had some really small external speakers, 3"x3" and they were 4 ohms, one sounded ok, two works great.  The RS speakers were 25.99 each, kinda spendy, but they work and are easy to conceal.

Ned