And yet the kids said they saw him wiping the cab down with a white cloth, IIRC, or at least something that appeared to be a white cloth as seen from their location.
So he didn't use his clothing.
he supposedly had Paul Stine's head in his lap.
I'm not sure how anyone would know that The Zodiac cradled Paul Stine's blood-soaked head in his lap. The kids certainly could not see such a thing from their location. There were no other witnesses.
I don't think The Zodiac would have taken Stine's head into his lap. I think he pushed the man's head and upper body into the seat well while himself sitting on the door-side edge of the cab's right front seat.
It's what I would have done.
He then could have swiveled left to cut the shirt cloth.
This whole narrative of The Zodiac shooting Stine from the front passenger seat and then cradling the dead guy's gushing head inside the cab isn't based on any established fact. It's based on a few lines of testimony from the kids who didn't go into elaborate detail. They never saw the cab arrive, and they never heard the shots AFAIK. They did not see The Zodiac arrive in the front seat and shoot Stine. They simply said that by the time they got to the window, The Zodiac was up front on the passenger side, doing something, after which they claimed he circled the cab, wiping it down. Nowhere in that description is there a depiction of what's been claimed, which is just baseless supposition and speculation.
To this day, no one knows exactly why The Zodiac cut the shirt. To prove he was the killer? Yes. But then there's this: if he cut Paul Stine's shirt and then circled the cab to clean up "clues", why
wouldn't he use Stine's shirt? Why would he use his clothing or a perfectly good handkerchief when he had, at that moment, a suitable object that was disposable and likely already soiled with blood? Would you use your clothes when you had in hand a piece of rather useless cloth? I wouldn't. I also don't think the Zodiac was much into other people's blood, which is perhaps why he wore the costume at LB: to keep himself clean of such bodily fluids. Perhaps it started out as a bib and some kind of head covering, like something one would wear during surgery, and then, The Zodiac being The Zodiac, the garb evolved into something a bit more symbolic.
(black doesn't show blood, by the way)
Regardless, I surmise then that whether or not he cut the shirt for proof of his involvement or to clean himself and the cab, or even both, he likely used the shirt to wipe the cab.
As for rending the shirt...to say that he used the keys is imo rather unlikely. We know the Zodiac carried a knife at Berryessa, and it is unlikely he would have approached the Stine scene without some kind of cutting blade. He was that ilk of a person. Could he have cut the shirt with a key? Possibly. Keys are often sharp in places and one doesn't need a perfect blade to get a running tear on cloth. But we know The Zodiac was into knives, so it is more likely to me that he approached the scene equipped with one, simply because he was a murderous brute with little disinclination for sharp blades.