I believe I've seen the same sort of thing, JFB, but can't recall what it was named; a search for "truck gun safe" brought up this craigslist ad from Harrisburg, PA, showing a 3-gun safe by Homak that mounts behind the rear seat; he may still have it (it was listed in Feb.), but at least it shows that they are out there:
http://harrisburg.craigslist.org/spo/589688279.html