Thankfully that was one job I never needed to do on our W115, as it appears that the car was assembled around that and the HVAC fans on this model. There has beeen some discussion about this in the past, but more often it was the fan access. As far as the stop leak product, there is no way to prevent water and/or product from moving out of the heater section and into the general engine cooling system!!

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150