This is easy. You already have the 2 bolts out that hold the pipe to the heater core, correct? Now go pop the hood, remove the mesh cowl that sits under the windshield wiper and then disconnect the hose clamp on the driver's side that holds the metal pipe to the rubber hose that goes through the firewall. Now you can either pull the pipe from the engine side, or push it from the compartment side. just wiggle it or turn it a bit to maneuver it enough to replace the o-ring. The passenger side follows the same protocol.

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1990 190E 3.0L