I just found this topic and have a related question. I will add comments that will no doubt be a bit late for your task, Jim, but it may help others. Several years ago I had to replace the headcloth because moths had set up camp behind it.
Firstly, my question - what was the style of fabric for the blind? Was it in matching headlining cloth or something else to contrast with the upholstery or carpet, or something neutral? It is not important in the scheme of things as one can be creative with this. A few years ago I entered my car in a concours (to find out how much work I still needed to do) and the judges commented on the incorrect blind material, but I did not get a chance to ask what it should be.
Back to what I found during my replacement task, which might help someone else, and to add to Ed’s and Graham’s comments. The route that the cord takes is this. The two headlining steel hoops ( easily made from 1/2" x 1/8" flat if rusted and the eyelet studs broken) have tapped holes to take the eyelets. There are 6 in total, 5 in the rear hoop and one in the forward hoop. These are to keep the cord following closely to the contour of the headlining. There is one above each end of the blind suspension rod (just clear of the rear window sides), and one in the centre between these. There are two more to take the cord around the downward curve of the hoop. There is a single one in the forward hoop aligning with the outer one in the rear hoop. From there the cord becomes obscure behind the headlining. There is a small hole through the wooden transom which fixes to the back of the sunroof box and the cord passes through this and above the cant rail to a small metal guide bracket above the driver’s head. A section of the weave is opened here to allow the cord through. There is a smart little art deco peg a little further along for the cord pull ring to hook onto.
You will need to install a pull wire through this overhead area of course, during the headlining replacement. There is nothing fancy about the provision of the holes for the cord - just open up the weave and pass the cord through. The little peg at the front is important as it maintains a fixed point for the pull ring to rest on without straining the headcloth and is the reference point for the blind upper stop position.
The roller is a conventional roller blind but with no ratchet, as you cannot operate it with a ratchet, the blind being upside down to conventional use. I had trouble finding a replacement roller blind, as they were too big in diameter to fit behind the seat. I eventually did and have kept it as a spare, as I managed to unseize my original. The diameter is important, as is the thickness of cloth to avoid binding in the gap. (If you use a suitable modern roller, you must permanently lock out the ratchet.)
Twin cords run from the pull ring through the cant rail cavity then through the first four eyelets. One then drops to the first side of the blind and the other feeds through the remaining two rings and drops to the other side of the blind. It is advisable to wind the roller to the lowest strength for rewinding as there is more force than you expect for drawing the blind up due to the resistance from the roller spring and friction through the eyelets. With one leg of the cord a bit longer than the other, you may find in time that the natural stretch makes the blind lean slightly to the longer side. To overcome this, I use a single cord doubled and attached with a clove hitch through the ring. This allows an easy adjustment later.
The eyelets are somewhat special - heavy section, small eye, integral washer, and 6BA stud. I don’t know of any available so if you are missing these, you may have to make something as an alternative. Personally, I would go for a 1/8" Whitworth. The washer is important as it covers the hole in the headlining where the eye passes through.
I used venetian blind cord and soaked it in strong tea to give it a colour closer to the head cloth.
That’s about it, but what should the blind fabric be?