Time for a Japanese lesson, since, hey, the company is a Japanese company, I live in Japan and my pronunciation is damn near perfect...
In Japanese phonetics, there are actually three characters to make Nissan in written hiragana (にさん) and katakana (ニサン). But only two in Kanji (日産).
The characters "に" or "ニ" represents the "ni" sound, pronounced "knee"
The characters "さ" or "サ" represents the "sa" sound, pronounced "sah"
The characters "ん" or "ン" represents the sound "n" which is just an "n" and is pronounced like the "n" in "can".
So, phonetically, it's correct pronunciation is knee-san, like we Americans say it. British pronunciation is TOTALLY WRONG!!!
Live in Japan, speak Japanese and read hiragana and katakana.