pretty easy cars to work with under hood. there is plently of mods for the engin/etc, and they take upgrades pretty well with stock internals.
HU is easy to work with, double din space. aftermarket radio is cage mounted, so you dont have to worry about kits/brackets, (you need a vw pocket for single din)
the trunk area is awsome to work with, back seats come out real easy.
getting through the fire wall is pretty easy on an auto, as there is a clutch knock out. im not sure on a manual.
running wire is fine, just takes some time, because its pretty tight under the carpet/panels. etc. i was able to run 0 guage no prob.
i do have a box loaded with 2 15"s that is formed to the trunk (any golf/gti) that i will be selling soon.