The first things that I would look into you be, the amount of current being drawn by the motor and what it is compared to spec. And also the condition of the blower motor resistor.
As far as the dealership being "more expensive" I too used to believe that they charge too much for parts and labour. However being a Technician for over 12 years who has worked in smaller garages for 10 years and currently a Toyota dealership for the last 2. Is that yes, parts and labour at the dealership do tend to cost more than at most other garages. However their technicians are generally better trained, which will lead to less diagnostic time spent on solving problems. And also their parts do last longer, If we look into your problem, your original motor lasted 8 years while this after market one has lasted 6 months. And the problem with excessive current may even be a problem with too much resistance in the replacement motor itself.
Therefore going to a dealership might not be a bad idea if you have had problems with improper diagnostic. Also I know at our dealership we are all willing to bend over backwards for all of our customers, especially new ones. Because in this industry the hardest thing to gain and the easiest thing to loose is a customer.
I hope i have been helpful with your blower motor problem, And I hope where ever you take it is able to find a correct problem with it.
Please let me know any solution that is found, Ild like to hear what is causing this problem.
Also if I have been helpful, please leave feedback accordingly. Thank you