Hi Serge.
Actually there is but one blower motor for the 'scorts in the US. As far as I know this is true for most if not just about all passenger/sedan vehicles. Minivans will typically have two blower motors; one for the back. Different sets of "doors," sometimes servo-controlled are all that are used to redirect airflow for the front defroster in sedans and vans.
High resistance or poor conductivity is what causes this problem. If you had a very low "volume" of sand slowly trickling through a very large pipe, that pipe wouldn't necessarily get very hot would it ? Well, what if you had a very high "volume" of sand rapidly flowing through a very small diameter pipe ? The friction increased by the pressures would make that pipe very hot wouldn't it ? When the material in those contacts corroded, that effectively reduced the diameter of the pipe. If the contacts were larger and their surface-to-surface contact area increased, that might've helped to mitigate or at least, further delay the onset of excessive temps.
This principal is why lower gauge wire (meaning thicker) is used for high amperage draw devices such as household vacuum cleaners. Pulling large amounts of power through a small orifice creates a lot of "pressure" or resistance. The material under so much "pressure" begins to heat up; or converts some of that energy into heat. With the contacts being corroded that effectively reduces the contact surface area capable of efficiently conducting current. Sweeper manufacturers will not recommend this in their manuals, but if you've ever tried to run an aged 12 amp vacuum cleaner through a cheap, thin extension cord, after some time you may notice that the cord will get warm and may even damage it's own insulation.
As far as the diagnosis, I wouldn't have labeled this a "TIP" if I weren't sure.
If you see inconsistencies and/or holes in the plastics of the wire harness, and/or you see that the contacts are corroded, then they need to be replaced.
In my case, there are about three holes between the two wire harness plugs. The most corroded contact is the one that has burned the huge gapping hole through it's compartment in the wire harness.