Since I had the new top w/glass installed I've only replaced the engine air filter a MAP sensor myself......but I can tell you that access to the engine compartment is the same with my glass rear window as it was with my plastic window. when I place the top in the "service" position.
IF you're thinking of getting the top with the rear glass window....I suggest you first get the exact dimensions of the glass from your top's manufacturer of choice, then cut out some cardboard to match those dimensions and place it over your plastic window.
I suggest this because the neg rap on the glass rear window is it reduces driver's rear vision. I can tell you it really doesn't. All you need to do is re-adjust your rear view mirror slightly.
The pluses of the rear glass window exceed the slightly smaller window size...IMHO
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98' Boxster
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