Power wires to various things like window switches, sunroof switch, interior lights are low. My windows take aaages to open and close (cleaned switches already), and I have to push my sunroof to help it open.

In the case of window switches and the sunroof switch, I get around 10v car off. A little more when on.
At the map lights which I'm trying to get working I get 0 off, 0.7 acc, 6 on (odd, shouldn't it be from battery?). This is from a solid ground reference all cases. Battery is around 12.6v.

What could be causing this? I've thoroughly wirebrushed all ground points thinking that might be it, but no difference. Its a problem with the power wires, not ground paths I believe.

Could it be corrosion inside fuse box? Corrosion in ignition barrel? Need advice.

 

Interior light on the roof is straight battery to ground (or at least it should be). Since it is widespread, fuse box? Measure voltage between BAT+ and the interior fuses first, should be close to 0 when car is on. Could always try measuring resistance as well, normally I would not recommend that without a mili-Ohm-capable meter but with that much voltage drop you should have an easily measurable problem somewhere.
The ignition switch is also a good item the take care of either way, so brush away. Pretty easy to get the contact cylinder out the back but be careful when taking it apart after removal.