Here's an ac vent behind the speaker - home made
The vents installed where the speaker would go-
The dash with the speaker cover installed- (the radio is just a dummy)
This is the assembly ready to mount in the dash - it uses the two studs from the trim to hold it in place
To build it, I made a quick CAD drawing of the mounting plate - the dark red lines are the opening in the dash superimposed over the part- wanted to make sure the plastic vents would fit.
Knocked this out on a waterjet, but it'd be very easy to make by hand too
I took some rod stock and glued it to the back of the grill in a feeble effort to not block the flow
I haven't figured out how to adjust the louvers yet. I'm not sure it is necessary. In Phoenix, where I built this truck, the AC vent usually stays where its pointed and comfort is adjusted with the fan and temp. We'll see.
In Phoenix, just these two vents won't cut it, so I'm also planning on having a couple vents hanging off the bottom of the dash. There was a Ford truck at the local SoCal Speed Shop that had them on hinges at each corner of the dash so they'd fold up out of the way for shows/winter. Might try that approach too.