First season, it's a bit that you have to parse out, but I think the story is that Kleya (probably) contacted Pak at some point -- I think the implication is that Pak was a Separatist during the Clone War. [You get this from Dedra during one of the interrogations I believe]
Pak, who was in the same kind of scrap-yard business as Bix, put Bix in contact with Luthen so she could sell parts that came from Imperial equipment instead of demolishing them. Luthen then (I assume) passed the parts to people like Saw so they could use it to maintain their stolen military hardware.
I think that's the logic-chain. I don't think Bix had any idea of Luthen's role in the growing resistance, he was just a fence.