Let's start at the beginning. What is your goal? Why do you need the MTBF?
Is this a customer request? Internal request? Or is your goal simply to improve the reliability of the product?
If your goal is to improve the reliability,
forget MTBF. You already know that the fan is the weakest link, so improve it and you do improve the product. Ford does not worry about MTBF or reliability testing, they focus on
DFMEA/
PFMEA and improving the demonstrated weakest links.