It is impossible for any automaker to manufacture all the parts that go into their vehicles. More often than not, specialized manufacturers will design and produce parts that end up being used by the automaker. These companies are known as OE Suppliers, and it is their job to manufacture parts for automakers to use in their factories when their vehicles are assembled. It is perfectly acceptable to use reproduction parts, but original equipment parts are the best at bringing your vehicle back to perfect form and functionality. The most famous and ubiquitous OE suppliers include companies like Bosch, Continental, and Monroe.
One of the most common parts to require replacement in your HVAC system is the blower motor resistor, so make sure to test it when diagnosing any heating problems.
OEM: 4E0 820 521 C, Item: 513113
OEM: 64 11 8 391 399, Item: 518685
OEM: 8K0 820 521 B, Item: 676811
OEM: 210 870 02 10 39, Item: 791548
OEM: 210 821 46 51, Item: 493868
OEM: 4B0 820 521, Item: 552121