I see. Well, there’s no seller on aliexpress who would happily recommend C4 for older vehicles ( before 2000).This is unfortunately common with c3's on older ones the relays go bad I have also seen bad cables.
The c3 got pretty cheap on aliexpress $150 or without cables.
You could get a new one and hope for the best.
If you want to to be on the save side get 's c4.
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Basically their answer is if I want a stable MUX for an old MB, then C3 is the way to go, if you wanna “try it” get the C4. So, now either I get a C3 with bad relays or C4 which can be unstable. Really confusing.