HA12240FP not responsive - what am I not seeing?

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Joined Aug 6, 2017
I'm trying to hack my Toyota Corolla 2006 stereo head unit. It outputs AVC lan out of a port on the back, which is a proprietary language communicated on a differential bus. I figure all I need to do is to hook that up to an arduino, learn the language and I'm rolling.
Unfortunately I'm having a lot of trouble getting my arduino to read the differential bus lines.
What I'm currently trying to do is copy the input/output circuit of a chinese aux cable which works the same way; arduino and all. Here's a rough schematic of what I've measured on this thing:

Pin eight of the HA12204 is an on/off switch, which the atmega simply sets to high on poweron (and then the microcontroller seems to leave it there forever). There MIGHT be some capacitors at play in this circuit that I can't see, but as far as I can tell these lines are not connected to anything else.

When I connect an oscilloscope to pin 2 on the HA, I see binary transmissions at logic low of ground and logic high of 5V. When I recreate this circuit with my own HA12240, I get ground. If I remove the 10k resistor, I can see a very, very small (like less than 10Mv) peaks that coincide with the messages that should be appearing. It's as if the thing is not getting the right voltage (but it is, and I've checked at least five times).
Here's the datasheet: https://elinux.org/images/0/03/HA12240.pdf
Is there anything obvious I'm missing? Anything else I should try?