1. 5 meters, http://www.usb.org/faq/ans5/ This applies to both USB 1.0/1.1 and USB 2.0
2. As far as VGA cable lengths go, I don't know of any hard and fast rule. Basically the shorter the better. If you have to run a cable of 5m try to buy very good quality cable, otherwise you'll surely get interference. Good luck. Oh I see what you're asking now hehe whoops anyway, I am sure that you'll get different answers for this but the center of the car (assuming no other wiriing is there) would be the "safest" from EMI but it's probably safe to run it with the power cables but really it may vary and experimentation is best fo rthis.
3. I don't know what IR device you're talking about, look at the specifications or get in touch with their technical support.
4. No I would really guess not unless your rear window is at a very odd angle or something, but again just try it out and see if there's any difference in how easily it locks on to satellites in both position. I'm guessing there won't be a big difference, if any.
5. I'm really sorry but again, test it out. If you buy high quality shielded cable you should be alright.
Good luck and I'm sorry that most of my posts just ended in "test it" but that's how a lot of it works. Every application can be different and there can always be other factors which are unknown especially when you're talking about interference. Good luck again, hope this helps!