A through Glass might work at VHF, more likely UHF, but frankly.. I"m less than impressed by 'em. And at HF they are a joke.
Another thing I did once with a car was design a "Z" Bracket basically make the / in the "Z" vertical like this | so you the bottom part went under the hood to a fender mounting hole, I removed the fender bolt and replaced with a slightly longer one.
The antenna was mounted to the top part which extended out over the fender
NO HOLES IN RIG
coax went through the firewall in an existing hole
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That was the bracket's shape
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