I had a problem with the Blinker stopping on hot days and sitting in traffic.
If the convenience module is the same technology as it is with my 95 E320 Sedan, then there might be a good chance that the riveting on the plugin posts are corrosive underneath, not visible to the eye.
You, say you soldered lots of cold ones, I think they soldered them in a wave and had perhaps a bad batch of products produced.
If you can, check the rivets and resolder them using a drop of solder flux. The flux does normally clean the corrosion, making it a better, durable contact.
I am almost convinced that the rivets are the culprits!

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Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert