Remote Key Fob (all Functions) only work when I'm INSIDE the car.

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I have a 2005 Nissan Sentra Spec-V (2.5L 4cyl Engine & 6-Speed Manual Trans.). My question is regarding the Remote Key Fob/Alarm/Keyless Entry. It's completely stock, & I want to keep it that way. I haven't had a problem until recently, & then all of the sudden my doors aren't opening with either of my 2 Key Fobs unless I'm sitting INSIDE THE DRIVER'S SEAT OF THE CAR pushing the buttons. I changed the batteries in both remotes (two times in fact to be sure), & still the problem remains. If you're outside the car it simply won't work. Not one of the functions on the Key Fob will work (Lock/Unlock, Trunk Open, Panic Button). I'm at a TOTAL LOSS for what's going on since it seems to have somehow lost it's range?! That's what's so confusing, because it DOES work inside the car, but outside the car, forget about it! I can't even figure out where the keyless entry's Remote Receiver (I'm assuming it works by I.R.) is in the car. I've looked on the dash & in obvious places to no avail. PLEASE HELP! I don't want to have to goto a dealer, I would prefer to keep the factory alarm. Thanks! Andy