So I had the common demister panel crack on my 03 STR. I decided buying a used part was a waste of money and buying new was ridiculously expensive. I went to Ace Hardware and bought some DevCon plastic welder epoxy to repair it and I am happy to report it is working great so far. This repair was done about 6 months ago and it has now sat through many hot/cold cycles including time in the sun. It isn't perfect, but there is only a small hairline crack visible from inside. The cracking/rattling sound I used to have is gone. All in all I think I spent around 20$ and a few hours. The adhesion this DevCon had to this particular plastic is insane. I picked the panel up by one end like a fishing rod after the epoxy set up and it didn't crack or break.

Here are pictures of my procedure and products I used:

Obvious break, it split right in the middle.



Lining up my joint for gluing. I used a newspaper underneath on each end to create the proper alignment.



After cleaning with rubbing alcohol and sanding it was ready for glue.



My first tube of epoxy was used on the center section and reinforced the sections known to break on the ends. I spread a TINY amount in the break before sandwiching it together.



My second tube of epoxy was used to slather the center section for added rigidity.



This was my "reinforcement" to prevent breakage at another known weak point.



The finished product with foam tape and set-up epoxy.



My finished joint as seen from the top. The mild discoloration wiped away with a little rubbing alcohol.



The plastic welder I used. I ended up using 2 tubes.



The foam tape I used. One roll is more than enough to cover the full panel.

 

Good repair I hope it holds up for you. I fixed my defroster panel 2 or 3 times the same way but the plastic just was so brittle that it kept breaking in different spots. Never did break at any of my repairs. I had to buy an expensive new one. I sure hope it holds up!
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I could not find a unbroken panel so I use a with a little modification a Lincoln ls panel, the plastic it made of appears to hold up better than the jags hope that helps

 

Good to know! I did not even think of that either. But you are right they must have used different plastics because my 2003 Lincoln LS has the original panel with no cracks or rattles.

My 2005 STR I repaired the panel twice and it broke/failed again and I had to buy a new one from Jag for about $200+!
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@benpearce2013 - Great write up with really helpful pics. Thank you, sir!