Just "bite the bullet" and install a new top w/the glass back window. Yes it's costly, and yes the rear glass is a bit smaller than the plastic, BUT the glass has heating wires for defrosting and your car already has the plug and panel switch (installed at the factory).
Your top probably has seen better days, since its probably original, making it more than 20 years old.
A new top quality top w/glass will cost about $1500 installed by an experienced installer.
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98' Boxster
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