I have held to using Meguire's Vinyl and Rubber Protectant on dashboards and have had really good luck with it. It does seem to moisturize well. Other than that, I would say shaded parking during the summer, or have the windows open. Even though you are in Minnesota, I still bet a closed car can hit 120 degrees. Here in Florida, interiors can hit 150 during summer. The dash pad in my white Comet was completely destroyed when I got it, and it was just a 9 year old car then. Most of pad cover was pieces 2 inches square.... splits everywhere. The heat kills these dashpads. Don't imagine MN cold does them much good either.
Dead on target there. In that respect.. our climate is even worse as our cars have to expand and contract between the two extremes. From 130+ to -30 interior temps takes it's toll on old and constantly de-plasticizing Vinyl parts.
Its a small crack, I doubt its weather related as it sits in climate controlled 99% of the time. Seems to expand from driving. Its hardly noticeable right now but wanted to get ahead of it so I wont need to go to a cap. It seems eventually I will have to though. It's a shame as the rest of the pad is perfect.
That'll surely slow it down a good bit. My guess is vibration and heat soak from the sunlight when you do drive it. Just remember that the plasticizer is probably nearly non-existent after all these years too which makes them shrink, warp, and crack much more easily. Maybe you could use some vinyl or plastic repair material and repaint it? Ideally you would reinforce the entire affected area from the bottom-side while it's off to reduce reappearance of the repaired area. I've never done it myself.. but have known many others who've glassed the undersides during similar repairs like this. Would be tough to overkill it if you want it to last. Good luck with it.
A dash with that size crack is the best kind of dash to put a cap on. Once the cracks are numerous and all fluted out....takes work to get a cap on there.
