It was most often pronounced "Hun-die". Terrible.
Then, as the company produced more and award-winning vehicles, native English speakers said "Hun-day". Not bad.
It is "Hyun-day" H sound plus "yawn" plus "deh". Not "day" more like "deh".
I am in my 22nd year of living in S.Korea.
New teachers from America, England, Australia, South Africa still mispronounce it "Hun-day". Those who've been here a year don't.
KIA, Korea International Automobiles, is the 2nd biggest car company here, and pronounced different than in English "gee-ya" but since it's name is an English acronym it feels wrong to say the Korean pronunciation when speaking Wnglish!
Note: i never had a Hyundai. I had a Datsun and two Toyotas before my Chrysler. Then here overseas i had only two by Korea's 3rd biggest car company, Ssangyong Motors (new name KGM): a Korando jeep and now an Actyon crossover SUV.