Firstly, are you sure its Mozart?

Try Youtube. Search Mozart and listen to a few pieces til you find the one. Add in "concerto" to find all pieces written for a solo instrument (not matter what the instrument is) accompanied by an orchestra.

Mozart wrote mainly piano concertos -  27 for piano, only 5 for violin, one for clarinet etc etc  

Its probably piano, and if youre not a classical music buff, its probably well known - is it in any ads or movies we might know?