Thanks for posting photos of your W108 heater. It's certainly much different from the unit in a Fintail.
If you can reinstall the heater assembly with the controls attached, it would make things much easier. I only had to replace a friend's heater controls on his '72 280SE, with AC, and it was a pain wrestling with stiff cables, while fitting tiny screws, clips and washers, through the radio and speaker openings.

Happy Motoring, Mark

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DrDKW