Alright, this post might be hella outdated, but I've seen some questions on this website on how to get the Merryweather car (because who doesn't love those bullbars all over your f*cking car).
So, as far as I know, there are/were two ways.
[The way I got mine]
A mission by Lester, called Stocks and Scares.
The job is level 70 required, so I got my friend MiniDudeOwnez (he told me and helped me finish the mission, so props to him) to start up the mission and to help me finish it. In this particular job, you drive to the docks (same one where you climb to the rooftop and steal a Buzzard to deliver to Lester) and head inside the building.
In here, you'll find various Mercenaries you have to take out, then proceed to run to the upper floor and hack the door (NOTE: Someone has to provide cover fire here) once opened, take out the rest of the men there and crack the safe while again one must provide covering fire. Once you have cracked the safe, run downstairs and outside and drive off in your Merryweather jeep (there will be several ones standing, including a fully white one) to the delivery point.
From there you will spawn still inside your Merryweather jeep.
[The other way I heard of]
Call Mercenaries/get someone to call Mercenaries and kill them and take the car. Downside, costs 7,500$ and does not make you money nor RP.