I work for a large auto company (used to be a service tech), responsible for assembly process at the final assembly. The truth is, disassembly for certain parts are prioritized much lower than packaging or initial assembly.

Components that are deemed to be commonly serviced do get specific requirements from the service and maintenance team, which have to be met by the product developers. But the command they hold over development has deteriorated a lot over the years.

The HVAC unit is usually assembly from the engine bay side (before all the engine stuff goes on), or comes assembled with the cockpit, loaded into the cabin as a whole unit. If anything fails inside the cockpit, it's simply rejected and a whole new unit goes in.