A few points: Not having a blower with a marine gas inboard is illegal most places. In Canada, you have to run it for four minutes prior to ignition.
There are two types, "inline" (and I recommend Atwood) and "squirrel cage" (I have never heard the proper name for this, but they resemble an old-style, hand-held salon hairdryer).
Commonly, the larger "squirrel cage" is mounted close to the exit vent and drops the vent hose (typically the same wire and plastic type as is used in home dryers) down into the engine cave. You want this to go on the CARB side of the engine, because the A4 is typically "fume-y" in this area.
The "turbo/inline" type seems to be more common in the compartment itself, and the hose goes unimpeded to the external vent cowl.
Why this should be is another question, but it's what I've observed.
Make sure the blower switch contacts are clean and use a 10 amp (I am attempting to remember here) glass fuse and biggish wiring (12 ga. minimum). The blowers draw a fair bit of current and the last thing you want is hot wiring buried in the transom.
A final suggestion. Consider the PCV aftermarket kit from Indigo. It cuts down on the fumes. I found that when motorsailing in a following wind, I would get engine fumes in the cabin, and so I ran the blower in such conditions and cracked a forward hatch to clear out down below. Good luck.
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