It could simply be an impedance mismatch on the serial line causing noisy comms. All I know is it's working in the car only when the PCM has power. Also, aren't the uveprom based ECMs all 160 baud? Is it possibly trying to talk to an LT5 ECM? Just a WAG.

I've been digging through the processor datasheet looking for port register addresses. I think the key in switch pin may be a good point of reference because it triggers a wake interrupt. I'll try tracing it back.