Hard to say. I don't remember where the OP said the modules connect together -- if it's the white WTB connector (J6), or the bigger black one (J1).
J6 is labeled TS1 & TS2 +/-. The traces from the connector are routed via an inner layer, and I can just make them out on the top-side scan, until they go underneath the passives near the SiLabs F530A (8051-based micro), where I lose track of them. There doesn't look to be optical isolation, though. I suspect that one wasn't the module bus, but maybe temperature sensors (hence, "TS").
But, if that is the module interconnect, then based on the peripherals listed in the micro's datasheet, it might be a LIN bus -- it's kind of the intended application. I'm not very familiar with LIN. It seems typical implementations are single-ended, level-shifted from around 12V to whatever the micro wants, internally. Unless I missed it, the datasheet doesn't specify anything beyond ~5V tolerance, and the +/- silkscreen would kind of imply differential signalling. It doesn't really add up.
Now, if the modules connect via the black connector (J1), then they go into what looks like a SiLabs 8642ED. Can't find much on that part.
From what I can see, there are four pins connected -- two at the top left and right, connected together; and two more that might be TX / RX, or a bi-directional differential pair. There are SOT (D)iodes between the connector and the uC. It looks like the trace on the second position from the top left (near the "PCBA" silkscreen) goes into one of the pins through a 4.7M resistor (R72). It looks like the other pin, near R90/C58 on the bottom right, is driven with a 4.7k resistor. The diodes could be clamps, which, combined with the high-value series resistors, could be limiting the voltage to something the uC can handle. That would make sense with differential signalling, but single-ended would be tricky without a common reference (ground) between boards. I haven't traced all the stuff, and it's a bit hard to tell where some of them go with the parts and silkscreen in the way.
- - - Updated - - -
Oh, also, based on the layout, and nearby "VBAT" silk, U2 looks like it might be a voltage regulator. If I were going to hack this board, I would probably start there.