This mission is the hardest mission I have done solely because of the time limit and I almost ran out and I took nothing but shortcuts, I have always failed when trying to drive on the GPS route. ALWAYS. Also if anyone can do this on solo and record it I will be impressed.
If you have a sniper rifle, you can take out a lot of the guys in the yard from range without starting the timer,as long as it never tells you that you have arrived at the location. If you ultimately enter the facility from the rear (the opposite side from the road) in a reasonably protected vehicle, you can then jump straight into the truck while the timer has only just started.
High Priority Case.
Do it in the daytime and steal a hellicopter from the marina hellipads, (or phonw for a helli if you've unlocked/bought any) land on the MAZE bank building and snipe about 8 guys, then land on the buildiing and work your way down. grab the package, then the hardest part is the enemy helli. Use cover and either snipe the pilot or the guys shooting from it. Gte back in your helli and fly home.