For service and parts needs, visit your dealer. You will receive genuine GM parts and GM-trained and supported service people.
Genuine GM parts have one of these marks:

California Proposition 65 WarningInstrument Panel Fuse Block
The instrument panel fuse block is on the underside of the driver side instrument
panel.
The vehicle may not be equipped with all of the fuses, relays, and features shown.
...
Heater Core Replacement
Heater Core Replacement
Callout
Component Name
Preliminary Procedures
Remove the Heater and Air Conditioning Evaporator and Blower Module.
Refer to Heater and Air Conditioning Evaporator and Blower Module Removal
and Inst ...
Exhaust Manifold Installation
Clean the exhaust manifold sealing surface.
Install a NEW exhaust manifold gasket (1) to the cylinder head.
Caution: Refer to Fastener Caution.
Caution: Refer to Torque-to-Yield Fastener Caution.
Install the exhaust manifold (3) and tighten the NEW nu ...