Before removing the dash,...
Before removing the dash, I sat in the car to determine the line of vision through the windshield. On this '32 Ford, it was in line with the mounting screws for the dash, so I used those holes as centerlines for cutting the vents.
When was the last time you were driving your hot rod, miles from home, and it started to rain? Wipers will keep the windshield free from water, but not condensation. A handy roll of paper towels will clear things up, but two hands on the wheel are always better than one, especially in a hot rod in the rain.
Our solution at Ionia Hot Rod Shop was to build a defrost box. This '32 Ford five-window previously had A/C and heat, but no defrost; so, we thought it was as good a time as any to add this third layer of comfort while it was in the shop being "freshened up."
The heater had been purchased years ago sans the defrost option, and the center heat/air vent was not being used, so we decided we would route that vent to the windshield via some fabricated louvered vents. I think this procedure would be applicable to any early rod with a thin dash. It won't be strictly defrost, but it will be better than nothing.
 |  Using the rolled edge of the...  Using the rolled edge of the dash as a guide for the cutoff wheel, I cut a 4-inch slot, making sure to stay tight against the flange without cutting into it. |  The size and shape of the...  The size and shape of the louvers was marked on an old oak plank (any piece of wood will do). This will be used as the hammerform for creating the louvers. |
 I made a die punch from a...  I made a die punch from a 1-inch strip of 10-gauge with a smaller piece of the same welded together to form an L. I ground that piece into the proper shape, grinding the tapers to the desired shape of the louver corners. |  I finished the louver with...  I finished the louver with some fine-tuning in the corners, using a cross-chisel shrinking hammer and dolly. |  A 1/2-inch carbide bit was...  A 1/2-inch carbide bit was used to grind the form into the wood. It doesn't have to be perfect, but should be very close to the shape you want. This was finished with a sanding drum and file. |
 An assistant positioned the...  An assistant positioned the dash against the wooden hammerform while I rested the punch against the rolled edge of the dash (flush side toward the face of the dash) and hammered out the first louver. |  |  Using the dash-mounting holes...  Using the dash-mounting holes marked earlier as centerlines, I measured 2 1/2 inches off each side, giving a 5-inch-wide area, and marked it with vertical lines. |
 The plates were trimmed to...  The plates were trimmed to fit and hammered into a 90-degree bend. One of these plates will go at the left and right end of each 5-inch air box. |  I put two center punches on...  I put two center punches on each of the vertical lines previously marked-one approximately 1 1/4 inches above the other-and drilled them out using a 1/4-inch bit. |  |
 I fabricated a pair of sheetmetal...  I fabricated a pair of sheetmetal deflector boxes to direct air to the dash. Each box (A) will be fastened under the dash. Two-inch flexible hose from the heater will feed air to the boxes via a 6.25x2-inch plastic defrost duct (B) from Vintage Air that will be screwed in place. |  |  Grooves were cut into the...  Grooves were cut into the upper cowl to allow air to pass from the air boxes to the louvers that had been added to the dash. |
 The center duct will be used...  The center duct will be used to direct hot air to the left and right defrost vents. We cut a piece of 10-gauge to approximately 5x4 5/8 inches, trimming one corner to clear the temperature sensor and blower. |  I used the cutoff wheel to...  I used the cutoff wheel to trim away the portion of tubing that wouldn't be used, leaving just the bend. |  Now all we need is hot air....  Now all we need is hot air. The existing heater needed to be modified to provide the defrost function, and needed to snake its way around the gauges, ignition, cruise control, and cowl vent. We used duct tape to close off the left and right heater ducts dedicated to heat and A/C, and cut off the raised lip on the center duct using a cutoff wheel and cleaned it up with a 3-inch sanding disc. |
 With this flange plate temporarily...  With this flange plate temporarily taped in place, the heater was positioned under the dash so I could figure out the best way to route the tubing. It's pretty tight under there due to the MSD box, cruise control canister, and our biggest obstacle: the stock cowl vent. |  Once I had the tubing past...  Once I had the tubing past the cowl vent, I added another inch or so of straight tubing, plus a small sweeping bend I found in the tubing pile, and finished with a flared end to accept the flexible ductwork. Plumbing the passenger side was easy after all that. |  Finally, the flexible hosing...  Finally, the flexible hosing was added to route hot air into the deflector boxes and through the vents to the windshield. |