I have a 2002 Taurus with the similar door-lock and interior light problem. It first occurred during a storm. Problem resolved itself after car dried out in a few days. Can't wait until next storm!!
My heater blower stopped working on intermittent speeds after the same storm (resistor block failed). Mechanic replaced block, and it failed again in a few days (after another storm). Mechanic found that blower was drawing excessive current. Replaced motor and new resister block.
I found a Ford TSB 04-15-3 that addresses the blower problem (basically, install a water shield and plug some holes)..
Still looking for answer to door-lock / interior light problem.