The straps are corrsses to help hold them in the right place and tried to be kept short to stop them stretching when your braking hard/crashing.Some more clarification please!
For those suggesting using the upper seatbelt mounts, how would you do this with a 4 point harness (where does the 4th point go?)
If mounting to the lower belt mounts (against the advice of 1st post) or to a cage, are you supposed to criss-cross the straps or not? What's the reason?
For a 4 point I'd pick the top rear mount.