While few would consider this device a proper repair, the cowl vent cover does make an effective stop gap for those on a tight budget. The cowl vent cover is just that, a cover for the vent opening. It uses a foam seal and spring loaded retainers to seal the vent opening from water intrusion. It does not fix the rust issue, but it will prevent further damage and keep your interior and your feet dry for those daily drivers out there with cowl rust-through.
The cowl vent cover does block the cowl's airflow to the heater and driver's side fresh air vent, so many owners keep their vent cover at the ready in the back seat or the trunk and only pop it on when the weather turns wet. The cowl vent cover offered by National Parts Depot (NPD) uses high-quality billet knobs and virtually unbreakable Lexan material for the cover. Pricing is $25 for the '65-'66 and '69-'70 model, while the '67-'68 Mustang cowl vent cover runs $31.50. Naturally, there's no installation labor for this DIY fix.
Cowl Vent Collar Repair Kit
On the market just about as long as the cowl vent cover, the cowl vent collar repair kit is a simple DIY fix for basic cowl vent leaks around the spot welded flange of the "hat," or collar, area. If you have major rust perforation on the floor of the cowl panel, this repair kit will not help. However, if you have just basic perforation around the ring/base of the collar, you might be able to make an effective repair using this repair kit. Included in the kit are two plastic molded collars and a tube of silicone adhesive. The plastic collars fit up into the stock cowl vent collar area and when properly installed/sealed will create a new collar area that will prevent water from entering the passenger cabin.
Our recommendations for installing include cleaning and prepping the cowl panel's underside to prevent as much future rusting as possible, then gluing the plastic cowl hats in place. These steps will require the removal of the driver's side fresh air vent and the heater assembly. We've been told by paint and body experts that silicone adhesives can trap moisture, so you might want to consider alternative adhesion solutions like panel bonding adhesive. You might also consider removing the front fenders and cleaning the cowl vent drains of debris. We've also seen people drill access holes in the cowl side to spray in rust converter and/or undercoating to help patch the area as well for a two-pronged approach using the cowl vent collar kit. The collar repair kit sells for $15 at NPD and is generally considered a DIY solution, however you'll be looking at an all-day job to get to the bottom of the cowls.
Cowl Vent Sheetmetal Patch
By far the most popular repair option are these cowl vent repair patch panels. The left- and right-hand panels are sold separately, but most shops we've talked to over the years tell us if a Mustang needs one side, the other can't be too far behind. Let's face it, with the labor involved to remove the car's front sheetmetal, pull the windshield, drill out 200-plus spot welds, and then cut out the rusty cowl vent area, do you really want to go through all that labor/cost again a couple of years later for the other side? We didn't think so, which is why these patches are usually done in pairs (just remember you have to order each side separately).