I know that late model defroster vents have the side opening molded into them vs the earlier ones. are they interchangeable or are there differences in dimension? I am having problems getting late model defroster vents to fit into an earlier chassis. They seem to protrude more into the dash then I think they should. I am having a hard time to get the air boxes to fit into the recesses in the cowl in front of the windshield wiper setup when I match them up to the late defroster vents. I have tried to put them in as one unit i.e. air box clipped to the defroster and as 2 separate pieces. Is it a hard project or am I missing something.

I know there is a passenger side and a driver side defroster vent in the later style. I assume that the earlier style was the same way.

Thanks,
Buck

This is a function of the lower dash. The early ones are on the dash, the late are incorporated into the lower dash on the sides. May be possible to cut down???

Hi Buck the lower heater control boxes left and right never changed. As you mentioned only the upper defrost ducts changed to accommodate the side vents in 1972 and the one off center defrost vent in 1971.
I bought a late side vent system and early 1970 system and swapped all the best parts together for my 914-6 restoration. The hardest thing to find is a good upper RS early defrost duct as its one year only. The ears that clamp it to the lower heater control box tend to get broken off when the spring clips are removed.
That said the upper ducts are interchangeable but can warp being at the base of the windshield making re-installation a bit fiddly. Assemble the complete left and right box / defrost nozzle on the bench making sure the upper and lower parts are fully seated together and carefully install the spring clamps that hold them together. Install only new Genuine defroster duct seals; the Porsche material is a special foam designed to compress and seal. Anything else is likely to impede the nozzle sitting correctly in the dash frame making them too tight to get the bottom box seated.

Hope all this helps.