In my Odyssey I've removed the middle row of seats, and plan to remove the rear row as soon as I have somewhere to put it.
My Civic I gutted:
-removed spare tire
-removed plastic from the trunk
-cut non-supporttive metals from trunk area
-removed tar from trunk, rear and front
-removed all audio (radio, front/rear speakers, antenna)
-removed rear seat and all rear plastic trim peices
-removed all carpet (from, back and trunk)
-removed all HVAC and ducting from under the dash
-cut non-supporttive metals from the front interior
-replaced rear quarter windows with lexan
-removed front windows
-removed door panels.
-removed a/c
-replaced power steering system with manual rack
-removed rear engine mount
-replaced exhaust with no cat/resonator/muffler
-removed all wiring no longer in use (dome light, hvac, audio)
I think thats it
Edit: Oh ya, the windshield washer fluid reservoir, hoses and nozzles
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