There's something that is attracting the spiders. Perhaps something
inside the door panel that you can't see. Or something inside that
the spiders can sense that you can't see. A nest of bugs under
the drivers seat maybe.
Perhaps it's the location that you place the car. Something about
the ground or building it's in. A dead animal that is attracting bugs
which are spider delicacies.
When you fumigate the area, be sure to get it down inside the door
and also under the seats of the car and in the engine compartment.
I park my truck in an open shed and sometimes it sits for days
before I drive it again. The roof of the shed has a nest of
bats
that gather during the day. Snakes are attracted by the bats.
One two mornings I came out to find a snake on the top and
hood of my truck trying to get to the bat nest. One was about
5 feet long and the other was a 3 or 4 footer. Most likely
non-poisonous but startling nontheless.