Being from a rural part of MA I grew up never taking a key out of the ignition, and not knowing what locks are for, ya know. Had a sub and nice stereo (like all kids do) in my lifted jeep comanche pickup, and used to go into cities and such. Well twice in Lowell (big city) the truck got broken into (was very easy to do through the rear slider) and the sub got stolen and so did the stereo. One time even my GF at the time's canon EOS SLR camera got stolen as well!! Not cool. One incident was when I was too drunk at like 2am, a friend drove my truck to a diner, parked out back and the stereo was up when he pulled in. Probably not good. Then we are walking out of the diner a while later, and my friend is from a city so he was a little more street smart I guess, he said he saw someone in my truck. IT was dark out and the windows were tinted and I was like "hell no dude, what are you talking about." Sure enough some little asian punk was in the truck taking the stereo out. I thought he was a bum or something trying to sleep in there. He asked me for a cigarette and I gave him one. Then my friend was like "dude, he took your stereo" and I turned around and the kid was gone. That really sucked.
Moral of story, don't get a sub, just stick with the stock system!!