Hey tim,
I know some people get roof consoles and others put them down near the gear stick and others under seats etc if they're the faceless ones. I think the 3100 is a fairly simple unit like my 3200? Mines in the din slot until I find somewhere better for it.
Do a search for something like "mounting UHF", this will either give you ideas of how to install it or possibly how to root it!.. maybe try "installing UHF", it's been covered heaps.