I have never done the job so I can't explain the procedure, but the pipes are disconnected as one of the first steps in the removal of the whole box. This indicates to me that they should be fairly accessable. The picture shows two screws attaching the pipe flange to the heater core. They are sealed with part # 000 835 37 98 (a gasket).

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician