1)First, drive in backwards with the cannon facing behind. Drive it all the way til you hit the garage wall or invisible wall.

2)You want to find a way to block off the left side of the tank (the side with the hatch)

3)Using the phone method (thanks Nales), bring up the phone menu and call anyone for a mission. You have to wait til the mission option is available before walking into the scripted zone. Walk all the way forward til you hit the wall. You have to be facing forward. Now press the cancel button, then the enter vehicle button nearly simultaneously--you have to 'roll' the bottons. You will teleport inside the tank, skipping the whole animation process. It will then show you driving in the tank or attempting to drive it in the garage. If you get inside the tank and it doesn't show anything of you trying to get in the garage, just get out and try again. You may have to retry it a few times.

Than to insure it

The insurance glitch is where you get to insure your new vehicle. Glitch your new vehicle inside the garage the first time. Next, destroy a personal insured vehicle you want to get rid of--I bought a small scooter because it's cheap. Get in your new vehicle while in the garage, bring up Mors insurance, and keep the cursor over the destroyed vehicle you want to get rid of. Now drive out of the garage. While it's at the black screen loading Los Santos or when the menu to recover vehicles is back up, confirm the insurance claim. It should replace the insured destroyed vehicle in the open world with your new vehicle that you're currently driving out of the garage. Now you have to get it back in your garage to complete insuring using 3 methods:

1. Just get back in your garage and pull out a different vehicle. --CAUTION AFTER READING ANOTHER USER'S POST: IF YOU DRIVE OUT, YOU MAY EXPLODE DUE TO AN INVISIBLE TANK IN THE SAME SPOT YOU GLITCHED IT IN. GO OUTSIDE YOUR GARAGE, DESTROY ANY TANK THAT'S STILL IN THE GLITCH SPOT, DRIVE AWAY AND COME BACK SO THAT THE DEBRIS IS GONE.
2. Doing the glitch again.
3. Have your mechanic to deliver a different vehicle when you're away from your garage.

http://www.gamefaqs.com/boards/73372...nline/67633407