Like Theo suggested...put the strongest flow of air across the compressor. This can be done by blowing the fan directly at the compressor OR putting the compressor directly in front of the air intake. Either way, you want the compressor to be in the better part of the air flow. The difference, at this point, between blowing in or out will be next to nothing.

If there is a better chance of rain getting to the fan hole compared to the vent hole...blow out to keep the rain from being sucked in.

Surely another member has one of these and can look at theirs and tell you how Mr. "V' did it.

Fish on!,
Noel