What I did in a couple of minutes with NVEdit was add Honest Hearts' rain to the main game. The result sort of works: that is I get proper rain with effects in some areas. Goodsprings for example works fine. However in other areas (freeside for one) only the clouded sky works, the sound effects and rain VFX wont.
I have no clue what I'm did wrong, so if anyone feels like taking a quick look at my problem and tell where I've been stupid and forgot something , I've uploaded the .esp (a test file that makes it always rain) here:
http://depositfiles.com/files/1896fotyu