The "Get In" waypoint/command yields erratic behavior from AI. If a vehicle is not spotted from waypoint's proximity, AI will go to closest vehicle it knows of.
This is tricky, as vehicles that can simply be parked "next" to a building can go unseen, and cause the AI to backtrack to an unnecessary amount of distance, to attempt to complete the mission with a vehicle that won't be able to solve the problem (a boat, when the next waypoint is on land, for instance). This is also frustrating when building night time missions, as decreased visibility only heightens the problem.
My suggestion is that a "search" subroutine be implemented after the "get in" waypoint trips, so that the AI looks for a usable vehicle in a larger area around the waypoint.