I don't know much about the 900 series, but I'd be astonished if the cars were ever available with only one of the mirrors heated; I've only ever seen cars (from various manufacturers) with either none or both heated.
Certainly my '93 850 has heated mirrors on both sides, and very useful they are, too!
I suspect both your mirrors should be heated; possibly the passenger side mirror glass was replaced at some time in the past with a non-heated (and cheaper) replacement, or the element / wiring has failed.
Given the width of these cars at that point, I think mirror glass replacement is quite a common occurrence!
Cheers,
AJ