Wall - The dBDRA defines a speaker wall as any subwoofer enclosure and / or baffle board that meets one or more of the following conditions...
Enclosures that are greater than 30 inches (76.2 cm) in height.
Any installation which results in a vertical clearance between the top of the enclosure and the headliner of 13 inches (33 cm) or less. (THIS CLEARANCE REQUIREMENT ALSO APPLIES TO ANY EQUIPMENT WHICH MAY BE MOUNTED TO THE ENCLOSURE.)
Wall Test - The judge shall use a mic jig (or other appropriate measuring device) to verfiy a vertical clearance of 13 inches (33 cm) exists everywhere between the top of the enclosure (and any equipment attached to the enclosure) and the headliner. The judge will then verify (if necessary) the height of the enclosure to insure that it is less than 30 inches (76.2 cm) in height.
Adittionally in street classes (St A and B) the normal use of the vehicle must be maintained.
So, in other words if you are competing in Street B (assuming that you are using 2-15 inch and 2 800 cu in batts) you cannot exceed the window line...
I really don't see why you are linking the normal use of a vehicle at the windowline??