Another simple achievement.
You will get the option to hijack the Knox security trucks after the mission where you sort out that annoying Naz guy.
These trucks are easily recognisable as they have Knox written on the side. Anyway what i did was just hang around North Point waiting for the trucks to pass and would hijack them and take them to a nearby garage which would either be the one next to the car dealer near the import place or Tran's garage but it will show up on your GPS where to take them.
Do 5 and you will get the achievement and also you will earn between $10k-$25k each time which will go towards clothing and car purchasing money. Also there is a gold stat for Hijacking so these 5 will also go towards that.