I could be wrong about where the "rain drain" passes. If your put water
through it, all of it should come out the bottom. Pour a glass and catch
with same size glass.
As to why you don't smell it as much driving, your car is designed with
flow through ventilation. The constant changing of the cabin air takes place
with the flow as follows: in at the cowl ducts in front of the windshield,
through the interior, and out of the car through the trunk. The way it
gets out of the trunk is through doors built into the forward portions
(under) of the rear bumper (at the sides). These are visible after removing
the lining in the lower outward trunk recessions. Since the flow is away
you are in effect upwind from the smell.
If the "rain drain" isn't your problem and there are no leaks underneath, then you will need to remove the forward wall of the trunk and expose the tank. I think the fuel separator is in the tank. These kind of details I'm not so good with at home. My first activity professionally would be to
verify the whole circuit in the manual.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician