I’m thinking to replace a failing turn signal (multifunction) switch on Corolla, taking advantage of it (the car) being currently unused. Requires removing two-piece plastic steering cowl. Last time I removed the cowl, it cracked where the screws hold it together. Any tips on avoiding this problem?
I think the only way to avoid cracking is to leave it alone… 
Ancient, sun cooked plastic is going to crack no matter what precautions you take.
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Heat makes old plastic less brittle. Do it while parked in the sun with the heat on full!
Did it break the boss on the upper part where the screw goes in to?
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I have had OK experience heating plastic with a hair dryer or, better a careful use of a real heat gun on low, keep it moving, stop and test before too long. You aren’t trying to melt anything.
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Um, take the screws out first?
Sorry, couldn’t resist. Are there also clips in that area that need to be pried away to clear the other side of the housing? I don’t see why it would break there otherwise if the screws are removed. Can you clarify?
Heat is the best suggestion… along with patience and understanding where the retention mechanisms reside.
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+1
While it might not be possible to avoid cracking of old plastic, gentle heating with a blow-dryer before working on it will soften the plastic to some extent, and that could help.
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I’ll try the hair dryer heat-it-first idea, thanks.
You say I should remove the screws first? … lol … I’ll grant I’ve made a lot of bone-head diy’er mistakes over the years, but I do try to remember to remove the screws before taking stuff apart. Plastic parts often still stick fast & need prying even after removing the screws. Clamshell configured held-held emote controls for tv’s are a good example. In my case the prying did part of the cracking & one of the screw bosses cracked as well while removing the screws. Like you say, comes w/ sun baked plastic.
If all else failes and it still breaks put it back together with gorilla glue. 