I think that the small amount of Ruin in every human allows Ruin to effect them if they are insane. I have no been able to find WOG on this, but I think it is laid out in The Hero of Ages.
We see three separate accounts of an unstable or insane person responding to Ruin's prodings. Quellion, Zane, and Vin's mother. (All emphasis mine)
Quellion actually placed his spike himself, as I understand it. The man was never entirely stable. His fervor for following Kelsier and killing the nobility was enhanced by Ruin, but Quellion had already had the impulses. His passionate paranoia bordered on insanity at times, and Ruin was able to prod him into placing that crucial spike.
Quellion's spike was bronze, and he made it from one of the first Allomancers he captured. That spike made him a Seeker, which was one of the ways he was able to find and blackmail so many Allomancers during his time as king of Urteau.
The point, however, is that people with unstable personalities were more susceptible to Ruin's influence, even if they didn't have a spike in them. That, indeed, is likely how Zane got his spike.
The Hero of Ages - Chapter 70: Epigraph
She once asked Ruin why he had chosen her. The primary answer is simple. It had little to do with her personality, attitudes, or even skill with Allomancy.
She was simply the only child Ruin could find who was in a position to gain the right Hemalurgic spike—one that would grant her heightened power with bronze, which would then let her sense the location of the Well of Ascension. She had an insane mother, a sister who was a Seeker, and was—herself—Mistborn. That was precisely the combination Ruin needed.
There were other reasons, of course. But even Ruin didn't know them.
The Hero of Ages - Chapter 74: Epigraph
And we know the mental strength of the person allows them to resist Ruin, even if they have a spike in them.
Each Hemalurgic spike driven through a person's body gave Ruin some small ability to influence them. This was mitigated, however, by the mental fortitude of the one being controlled.
In most cases—depending on the size of the spike and the length of time it had been worn—a single spike gave Ruin only minimal powers over a person. He could appear to them, and could warp their thoughts slightly, making them overlook certain oddities—for instance, their compulsion for keeping and wearing a simple earring.
The Hero of Ages - Chapter 75: Epigraph
So I think it is safe to say that when someone lacked said "mental fortitude", they lost the ability to prevent the part of Ruin used in their creation from influencing them. And so Ruin was able to speak to them by virtue of his own power, instead of needing to augment it (similar to how Hemalurgy arguments an Allomancer's power, I think) with a spike.
Regarding the presence of Preservation and Ruin within humans.
Preservation's desire to create sentient life was what eventually broke the stalemate. In order to give mankind awareness and independent thought, Preservation knew that he would have to give up part of himself—his own soul—to dwell within mankind. This would leave him just a tiny bit weaker than his opposite, Ruin.
That tiny bit seemed inconsequential, compared with their total vast sums of power. However, over aeons, this tiny flaw would allow Ruin to overcome Preservation, thereby bringing an end to the world.
This, then, was their bargain. Preservation got mankind, the only creations that had more Preservation than Ruin in them, rather than a balance. Independent life that could think and feel. In exchange, Ruin was given a promise—and proof—that he could bring an end to all they had created together. It was the pact.
And Preservation eventually broke it.
The Hero of Ages - Chapter 54: Epigraph
This shows that life had a balance of both Preservation and Ruin before mankind was created.
Ruin confirmed this when he spoke with Vin.
Ruin shook its head, standing with hands clasped behind its back. "You still don't understand, I see. You're all on my side, Vin. I created you.
[Ruin] turned and began to walk quietly from one side of the cell toward the other. "You are a piece of me, you know.
"You couldn't create it yourself, could you?" Vin asked. "The world, life. You can't create, you can only destroy."
"He couldn't create either," Ruin said. "He could only preserve. Preservation is not creation."
"And so you worked together," Vin said.
"Both with a promise," Ruin said. "My promise was to work with him to create you—life that thinks, life that loves."
The Hero of Ages - Chapter 57
While I would not put it past Ruin to mislead Vin, I think coupled with Harmony's confirmation it is fairly solid.