I suspect the ethics or desireability of leaving a firearm in an unattended auto has much to do with the local environment of the firearm owner. I certainly would be hesitant about leaving a firearm in a car in the vicinity of downtown Los Angeles for instance. Where I live however it's common to see rifles in pick-up truck racks and it's a reasonable assumption there's a pistol in there somewhere as well. I normally have two handguns in my car and not unusual for me to have at least one rifle in my trucks. Of course I have three rifles hanging in my garage, I don't lock my house, and I leave keys in all of my vehicles for convenience. Don't be too quick to judge another's security concerns or lack thereof by your own particular environment.
As far as a "Truck Gun" and assuming a handgun as that seems to be your need there are many options from various online vendors as well as I assume local sellers. Any used Ruger, S&W, or Taurus revolver in .357 or greater in reasonable shape and reasonably priced would serve well. I'd also think one of those sinfully ugly but evidently functionally good Hi-Point semi-auto pistols in .40 or .45 would also be good choices. Very inexpensive, easy to replace, a lifetime warranty, and very ugly make for a gun you won't feel bad about losing through theft, damage, or (Heaven forbid) taken into custody as evidence if you ever have to use it to protect yourself.
(Those comments and advice were worth 2 cents but given to you free! What a bargain eh?)
P.S. If you need to refinish your choice in a quality but inexpensive finish I recommend Lauer Weaponry Duracoat. It's easy to apply, inexpensive, and makes a nice finish. A complete kit with one color will set you back all of $50 plus shipping and refinish 3 - 4 handguns.