Note: You can 'lock' the factory belts quite easily with NO MODIFICATIONS!

1. Move your seat forward one inch.
2. Put the seat belt as you normally would.
3. Grab the shoulder belt just above the buckle point at your waist and pull up sharply to tighten the lap belt. Hold it tight.
4. Using your other hand pull out on the shoulder belt and then let it snap back in place quickly. This SHOULD lock the belt. If not try again, you will feel it lock.
5. With the belt locked move your seat back to where you had it, this will pull the belt even tighter. Same as you would have on a four or five point harness.

Enjoy a more spirited drive without being thrown about.