What is this?!?!? and a tach question...

I removed my instrument cluster and I found this thing. It's mounted on the drivers side front quarter. You can see the fuse box right in front of it. I ask, because the bottom ground wire wasn't ever attached to it. I just attached it now, and I don't know what it is for?
Nice place for a speaker too huh? :) Don't ask me?? It was there!

Also, for my tach... It says to "attach the white wire to the dimmer wire". Well... There seems to be like 6 of them! Which one would I attach it too?
See... When I hooked it up the first time, I just thought... "It's a tach, I don't care about the dimmer anyway." So I left the wire bare. Then I noticed it had no light at all! so maybe the dimmer wire has to be attached for the tach light to work at all??

O.k. thanks! Any and all help would be appreciated! :)

The "What is this" pic...

Whatsthis.jpg