The lights have a small vent that runs to a pipe with a filter and it's possible that it could be clogged up with dirt or the filter has moisture.
6.1.01 Headlamp Unit
1. Remove the front apron undertray and remove the
complete nose assembly.
2. Release the headlamp plug from the body clip and
disconnect the plug.
3. Remove one bolt from the front headlamp mounting.
4. Remove the main beam lamp access cover and
loosen the rear headlamp mounting bolt.
5. Disconnect the front and rear headlamp vent tubes.
6. Lift off the headlamp assembly.
7. On refitting the headlamp assembly, carry out the
headlamp alignment procedure.
I suppose you could try the wheel well. If you look under the front wheelarch there is a black plastic cover.
If you take off the wheel and undo the three screws at the bottom front edge of the cover and then two more screws that are located going up the back edge of the cover then you can kind of bend the plastic cover back and take a look to see if you can gain access to the filter.
You don't need a lift (hoist) for this but make sure to use a good jackstand as the car is heavy and shouldn't be supported on a jack by itself.
With car standing still the headlights may mist up.However when travelling forwards,air pressure in front of the windscreen forces air down the 2 pipes forward,through the 2 filters into the h/lights then out through exits (low air pressure) in the undertray.Most likely the filters are blocked(the should be changed every 15/20000 mls.on early db7s to 96 they are located above each inner wing so only bonnet up and side covers off.On later cars both are behind the wheelarch cover so under the wing,l/h at rear of wing ,r/h at front(dont ask me why!).These filters are only inline petrol filters from many cars of the 80s and 90s and can matched in car factors,cost no more than £5 each.Getting the inner cover off on later cars is a struggle and will need the front wheels off.
While doing this it is probably worth checking that these pipes are not kinked or blocked and indeed exit through the undertray
the easier way or removing the comdensation is to remove both high and low beam bulb carrier assemblies and leave the housing right in there, then blow compressed air in to get the water to come out. Then do as solardon said, the filters are probably plugged, or full of moisture. They grab condensation from the Air conditioning system, as they are directly piped to the top of the HVAC box in the car. Make sure your A/C drains are clear.....that will give you this issue as well, and it's very common to have at least one drain plugged, often by the tranmission tunnel heatproofing blocking the drain tube on DB7 Vantage.....
Yours like mine ( 1998 ) has pipes under wheel well, but 1997
used to have them under bonnet,easy job to change filters and
clean out pipes.I will be putting mine back to the 1997 way.Will
involve cutting pipe ( making sure it is the right one ) as it goes
through wing from under bonnet.Joining new pipe and filter to it
and fixing to pipe which comes back under bonnet.Leaving all pipe
and filter under wheel well,no point taking it out.