Well I got a bit of progress on this. I took the bits to a Plastic welder and he said that the dash was polypropylene and in 25 years, he had yet to find something to stick to it and sure enough he could not weld the ABS plastic of the case to the dash fascia.
So anyway, this meant that I would have to mount the monitor into tthe dash like a head unit. I struggled to come up with an idea to make sure everything was in the right position as the Toyota radio is wider than a DIN unit, but the rear where the mounts are are DIN sized. This meant that there was no reference point to mark where the side wings would need to go in relation to the dash fascia.
Anyway driving home from work yesterday, I had a brain wave. I mounted the stock radio back in the fascia and it fitted in nice and snug. I then put a steel ruler up against the back of the head unit and marked the fascia as this is where the mounts are located. Then I positioned the monitor case where I wanted it and transferred these marks onto the monitor case. This was where the side wings needed to go.
I rounded up a couple of side wings from a plastic LCD mounting fascia I never used and visited my plastic welding shop again this morning and he did this:
One side was really easy as the mount needed to be hard up against the edge of the back side of the case, so we just held it together square on the bench and welded it up. Then we used the head unit as a guide to get the width right and welded the other side on. It was a bit more fiddly because of the shape of the case and we decided to simply weld in in some filler pieces rather than cut notches in the side wing I had.
Anway I tried the thing for fit and it lined up quite nicely, but I had to cut a fair bit more off the bottom to get it to fit.
Here is the dash where it had to fit into:
I had a bit of a problem in the bottom left corner and I trimmed a bit of the circuit board. Also there was a case mounting screw too close to the bottom edge which got in the way so I could not trim enough of the bottom of the case. I ended up grinding a fair bit out of the brace above the HVAC controls. Probably should have just cut the whole thing out.
Anway, Here it is all sitting in position:
I was pretty worried about how I would secure this to the radio mounts but there seems to be enough room to get into it frrom the outside to get some screws or bolts.
Anyway, there is a bit more work to do to fill the sides in and get clearance around the buttons, but I think it will work out quite nicely. It will be much better than the steel DIN LCD mount I was using once this is done.