hello all,its maxima-magician here.
Im a long time reader,not much of a poster,
So onto the discussion......
ive owned for years and sold four 3rd gen maximas and a nissan truck so i know mine,
heres my rain leak experiences with 2 out of 5 nissans.the other 3 were dry

1.my old 91 se had a jammed up moonroof and a towel stuffed in there, it was actually a clogged moonroof drain where it meets the antenna drain at the y connection not too bad too fix maybe the towel messed up the tracks so i put in a cheap used whole moonroof and track.
The car was also missing the plastic cowl shield under the wiper arms which was full of leaves,so i vacummed the out and pulled the blower motor under the glovebox and clean more leaves etc.,after all that the car was dry and clean.Then a while later-the passenger floor mat would get wet only when it rained alot so i knew it was rain,well after years of keeping a small towel under the glove box.So then when i finally sold the 91 i pulled off the side windshield trim and added sealant to the sides and top windshield trim piece which was partly loose as well and since i replaced the lost cowl plastic but never re-sealed it-
(i think theres supposed to be sealant there on top at least where it meets the glass)any way its staying dry and was not heater related,the rain was either getting in from the loose top windshield rubber trim and running down the a-pillar or maybe the extra side sealant helped or the cowl top now being re sealed to the glass fixed it.
The point is Dont go chasing heater cores always

2.my 93 d-21 truck also had a rain leak similar to my cars but i never bothered to try extra windshield sealant under the trim,when i sold it i left a sticker on the heater cores box stating its a rain only leak,not heater core
it would get a little wet under the passenger carpet so i made it liftable by loosing the trim and would keep a towel and only have to change it after heavy rains ,like the car