electrical problems after it rains

[LightlyToasted]

New owner of an '07 4.8i. Twice now I've experienced electrical malfunctions the day after the vehicle has been out in a heavy rain.

Symptoms:

• "transmission malfunction" message, warning that a gear could be selected without the brake engaged. Typically clears fairly quickly
• "electrical system malfunction" message, saying that further driving is not possible.
• CEL with various fault codes. Usually U0155 "lost communication with instrument panel cluster (ipc) control module", and sometimes P110F "Ambient Temperature Sensor Faulty CAN Signal"

Battery is only 1 year old, and load tests fine. Voltage is 12.35 with the ignition off, 14.35 with the ignition on, and around 13.7 while driving. I know 12.35 is a bit low, but the battery will accept and hold a charge no problem.

I suspect that the rain is getting into a control module and shorting out a bus. Forums say this is quite common with the E60 due to a blocked sunroof drain causing water under the spare tire to short out several control modules, but haven't found any similar issues with the E70.

Any suggestions where I should look?