I seem to have them more when I drive my own one. Occasionally if you blow one up they'll spawn like mad also.
Other than that it seems very rare to get one, it may have something to do with the amount of valid people in the lobby as I often do invite only with a few people and barely see em unlike when I'm in full lobbies where there more frequent.
They spawn on the nearest road, or whatever the game deems a road. Even if you're on top of Chiliad or out over the ocean, it'll take whatever's closest.
As for when they spawn, that's pretty ambiguous. All that I know for certain is that armored trucks, crate drops, and Simeon export cars are all treated as random events, and only one can exist in a session at a time. Meaning if there's a crate or export car on the map, it needs to be cleared before an armored truck can spawn.
I was playing a game with my friend last night and it spawned at the same spot over and over >> down the east side strip in the Tataviam Mountains and Ron Alternatives Wind Farm --- dunno if it was glitched but every time we stopped one another one would already be driving around in that same strip not far away from us
I notice they spawn more frequently when there's more people in the lobby.
I had 2 spawn right next to me but one was floating above the map cause it apparently spawned on a roof. Eventually it floated down to ground level then I blew it up, Immediately after that one spawned up the road a little ways. I got that one, too before my 3 star even started flashing.
Then when I look at my radar it said 4 other people attacked armored trucks at the exact same time.
But then they stopped super spawning and everyone decided to resort to griefing instead for entertainment. The lobby got all glitchy and I had to leave it was terrible.
Got back from Oregon 07/05/2010. Wasn't gonna leave but then I saw this: http://img149.imageshack.us/img149/9075/sam1064.jpg psn: EternalGST