Had same question. The answer is vague Cooper's web site -- what are the select retailers? -- but the person that answered my call from the rebate center said that all major retailers like Walmart, Amazon, Costco, etc qualify.
My last (and only) rebate experience with Cooper in 2015 was positive. Got reward advertised faster than expected.
2014 Mazda5 Sport van 2.5L I4
2009 Hyundai Sonata GLS 2.4L I4
2002 Subaru Forester L 2.5L H4
2001 Chrysler Sebring LXi sedan 2.7L V6