in the late 80's-early 90's, some peoplewould run small guage all-thread screwed into the back of the h/u and bolted through the firewall (this works extremely well)
buy a guard dog
razor blades can be hidden wherever fingers go
disposable cameras can be pulled apart. flashbulb leads spliced&connected to aluminum tape strips strategically placed in tight spots, not only will it shock the piss out of a thief, but it will flash with the bulb still in the circuit. multiple cameras and contact strips placed in several inconspicuous spots can really confuse, disorient, and deter a thief from proceeding.
I have really dark tints 15%. Even during the daytime, you'd really have to dry hard to see what kind of unit I have. Also, just general common knowledge of your neighborhood or the neighborhood you are going to. Sometimes I'd rather pay the $20 to park in a paid lot than run the risk of someone breaking into my car and stealing my $400 head unit.
Well, if you dont have insurance to cover for the theft of your unit,Id keep the stock POS in the dash, and install a unit under the dash with a coverplate, or throw it in the console, or in the glovebox accessable by remote control.Other than that, My insurance covers my stuff