Take some photos of the process if you can! I'm considering doing something similar for my 245. My window traces look black which I take as indication of the same sort of corrosion you saw.
Any issues avoiding scratches when removing the old defrost grid?
I shall do so and post them. When looking in from outside and comparing the grid wires in my car to the wife's 1980 w/replaced glass, I could clearly see a difference in mine. Spots that were discolored, varying thickness in the wire.
I had no troubles working the old grid off with the single edge blades. It is cramped and uncomfortable working in the rear seat and getting in/out in the 2-door for an old guy like me. But there is no alternative unless you remove the glass. I am not sure working on a 245 will be any easier or more pleasant - just different.
If you are asking if the glass got scratched-up while peeling off the original grid, the answer is no. But you have to be very detailed and go over each grid line a few times with the blade at differing angles to make sure you get it all off. Leaving anything behind but clean, clean glass is bound to mess up adherence of the new grid wire kit. I even scraped between the wires because after all these years there was a film of build up that seemed best removed this way. My final step was a good cleaning with Windex inside and out. The kit comes with some form of pre-install cleaner as well.
Be aware that even after the scraping is done, you will likely see something of a line remaining. You cannot feel it, it must be inside the glass itself. No idea what it is or why it is there. An FAQ at frostfighter.com says "The old grid will come off with a razor blade but there will be a brownish red tint in the area of the lines and that?s in the glass so it is not going anyplace." This is consistent with what I am seeing. I am a bit concerned that between these and the new grid wires there may be some reduction of "view" through the window. But I really think I am over-thinking it. We will see. There is an FAQ that speaks to installing the new grid wire by wire and that could be a solution to this. But the spacing they use for the wires is not identical to those in the car and so it is not clear this would produce results that adequately match their side bus bars and that would be a big problem. So I am plunging ahead with a "standard" install.
I also had to get access to the existing power/ground wiring. I did not want to rewire everything and to use the existing switch on my dash. The power "wires" are actually part of the tab that is glued to the window and they are not simple wires but thick, copper strips. These connect to the wiring at a connector located behind the "C" pillar covers adjacent the rear window. Ideally, I would have removed them and connect inside of there. But these plastic parts are old, brittle, and already cracked. I checked and can find no replacements for them anywhere. So I decided to leave them be, cut the copper strip as close to the tab as possible, and connect to that somehow (either soldering or some form of connector - still not sure). You can gently pull on some "slack" in the strip by slipping a small screw driver or other tool into the space where it is exposed and prying gently. I then cut through the strips using a dremel tool with abrasive disc installed. This worked well, was controlled easily enough, produced no sparks (although I was prepared for them if it had), and did what I needed done.
I now await the kit I ordered (1212-3450-L-STK). I also ordered their #2745 Tab Adaptor Wire Harness. After much special checking back with their tech support I determined that this was the most optimal kit for my use in this car. Sadly, this kit has both power and ground tabs on the same side and so I will have to deal with that somehow - not exactly sure what I will do yet. To get a result that they tell me will not overload my fuse/wiring, I will cut and install the grid in a trapezoidal shape that mimics the window's shape. This will maximize the overall length of the grid wires and this will increase the resistance across the entire grid and this will lower the current flow so as to not blow the existing 16 amp fuse and wiring. At least that is their theory/conclusion. Both of these issues can be addressed using a different kit (1210-2740-STK) but it results in a smaller grid area. This may be ok but I was striving for something more optimal.
Of course, all of this is specific to my car and a 245 will likely be different (but the same considerations will apply.