[q-authority]

I think people are making this far too complicated. After replacing my '85 MKI with an '87 (some dirtbag t-boned the '85 at an intersection) I realized how compromised the air intake system was on the '87-'89 models and devised a relatively simple workaround. Since I did not want to place any sort of new filter in the engine bay, simply because of the heat (and rain water), I had to come up with something else.
In the end, the solution seemed so simple I was surprised I had not seen anyone else try it already. For starters, temporarily remove the felt covered plastic partition that fits over the rear taillight assembly and then permanently remove the long stupid plastic air box/tube that connects the passenger side air intake funnel to the driver's side air filter box. You now only have the air intake funnel, up to the back of the electric radio antenna, on one side and the air filter box on the other. You can now reinstall the felt covered plastic taillight assembly cover piece.
This alone will help airflow to the filter box on the other side of the trunk, as the plastic connecting tubing is quite restrictive. But we won't stop there. Now remove the air filter box, from the driver's side corner of the trunk space after first removing its felt covered plastic trim piece. Temporarily remove the air filter and the cover piece that holds it in place (the piece that connects it to the engine bay air intake pipe). Now closely examine the structure of the rear piece of the air filter box (the piece that normally faces the against the driver's side trunk wall). After removing this piece from the trunk use a dremel cutting wheel, or similar device, to cut away all non structurally necessary pieces of this part of the air filter box. Leave only the portion that is necessary for retaining it to the trunk body and for properly holding the air filter in place when it and the compressor side of the air filter box is reinstalled. When you have cut away as much material as you feel comfortable with, reattach this piece of the air filter box to the trunk body, reinstall the air filter, and reattach the compressor side of the air filter box with its three clips.
You could stop there if you want, as you will have considerably better airflow than previously, but there is one more step that will help even more. Remove the passenger side felt covered plastic piece that fits over the electric radio antenna and trunk air intake funnel. Target the large air intake funnel that directly attaches to the passenger side air intake vent bellows. Cut away, from the top side of this piece, whatever size piece of plastic you feel comfortable with. You will now have free airflow from this new vent and from its original connecting hole, located just past the antenna in the corner of the trunk. Do not cut away any bottom portions of this funnel piece (or remove the whole piece for that matter) as there is always the possibility of water getting back into this section and it contains a drain pipe located just behind the radio antenna. If you leave the two felt covered plastic pieces off you will have as good as or better air flow than will ever be required by the engine. You would now have to really jam the trunk absolutely full of stuff to really interfere with it that much.
You can reinstall the two felt covered end pieces, but I have found that this will reduce the air flow somewhat. Since I don't really care about having them in place I have left mine off and have gained some slight extra trunk space in doing so.
However, when I finish with my '89 S/C project I plan on using a hole punch of some kind to place air vent holes in both end pieces (in a pleasing pattern of some sort) and reinstall both of them.
Either way, you will now have fresh cold air to your engine air intake and as much as it can use according to my testing. In fact, I also determined after having performed this that the K&N filter designed for the trunk mount air filter assmebly is not as efficient as the stock air filters. That's correct!! Sorry K&N, but your design for that placement sucks. Try it yourself and see. The results will be a bit more than obvious. Once I figured this all out I tried like hell to find a TRD air filter for the application, which was a cleanable filter, but with a stock air filter design, but unfortunately they have not been available for some time. The reason why the K&N does not work as well is due to its smaller overall surface area. The stock filter also usually has much more surface area that the filters used for mods that are placed inside the engine bay and there is now no worry about keeping it dry as well.
My '87 runs incredibly better with this mod done and I can’t wait to do it to the ’89 S/C.