Just a simple mod of the FBI Washington. Removed the center pillar and set the back doors up as suicide doors (they open backwards). Moved the door handels and removed those unsightly "badges" on the boot and bonnet.
oops, sorry for the slow reply.
This will spawn anywhere the FBI car spawned before.
Off the top of my head, there's one behind that record shop in downtown where you take lovefist and there's that crazy stalker.
HOWEVERZ.. the best thing to do is to download gtasf which allows you to add car spawns to VC. You could have one of these spawn outside your mansion then.