You are grounding out some part of or a good portion of your car's electrical system.

i believe every respondant thus far has given you accurate info and they all point to a phenomenon known as "insulation resistance." Whether it is the Hall effect switch (which sits next to the hot engine and there fore can cause insulation to 'breakdown' and drain the voltage to your spark plugs) or the plug wires themselves, or the signal to the fuel injectors, or some other wire, you are losing the voltage that forces the current (amperage) to some electrical device; this is commonly referred to as a "ground." If your wiring harness appears good, I would still look around for chaffed wires in the cable run, particularly somewhere that water can run to (as when you wash it).

Some parts of your electrical system are intentionally grounded to the frame. This 'grounded' part is the part of the circuit that is on the 'return' side of the load. The live or 12V side of the load should never be connected to ground. Water can act as a conductor and ground your circuit to the frame of the car. Also, worn insulation (and it does not have to be visibly worn) will leak voltage into the air and cause a weak or reduced voltage which will result in a weak spark. The damper or more moisture laden the air, the more voltage that leaks to air, and hence the weaker the spark.

my .02