the drainage is fairly simple in design. The external fresh air vents channel air into the black plastic box screwed under the dash, with a sealed fan inside to provide ventilation on these late type 3's. There are 2 drain tubes at the base of the box which drain water through the front panel onto the chassis. These are usually blocked leading to backflow. Disconnect and use a wire to clear them, have a bucket handy to catch retained water, dirt, mud, glass chips etc.
There is also a drain hole at the top on the drivers side under the external vent which blocks ( you can access from outside), leading to rusting at the front edge where water collects. ( Check under the bonnet at the central hump, lift the vinyl trim if you need to).
The plastic fresh air box has a rubber seal around the top edge, which is a pain to replace but if the drains are not blocked and there is no significant rust holes, you will need to. The earlier cars had metal boxes which rusted out, so atleast you don't have that problem to deal with,
Pat.
BTW, where are you? I have an old adjustable universal rack, not sure what type you are after.