Hi Ken,
Get yourself a good step down inverter.
For a stereo go with a marine grade system
There are several manufactures, check boat/marina shops
Near you or surf around.
As far as mounting under dash- not much room.
But , you can use a marine bracket ,similar to a cb radio bracket
Just make sure you use the rubber bushings on the mount
At the radio. You could also mount same radio in a 5.56mm
Ammo can under dash and close cover when not in use.
Or get a remote version, mount radio under seat near battry box
And place remote somewhere on dash, And finally an easier set up would be an iPod/iPhone with radio app
With mounted external speakers.(no need for antenna etc)Power
Your iPod from a cigarette lighter you have connected to your inverter.
I use the iPod method,