I've just looked at a couple more things. I followed the trail of the negative cable from the battery. One goes directly into the fuse box and the other, just as you mentioned
, goes into the engine block. Just below the air intake manifold. Very hard to access. I couldn't actually see were the cable is connected but I inspected the visible surroundings and I found this:
http://pwp.007mundo.com/chsoto/DSCF0586.JPG
I noticed a white/blue compound that looked like corrosion that had fallen on the starter motor (lower circle) and also some on the engine block itself (it looks yellow because I had the lantern pointing there).
Since I couldn't actually see what was down there I decided to take my camera and take a couple of pictures of the area from below and I found this:
http://pwp.007mundo.com/chsoto/DSCF0588.JPG
The horror! Now, I still can't quite make out where exactly is the ground wire connected to but, from what I could feel with my hand, it shouldn't be more than an inch away from all that corrosion. From what I can make out I think it is the "red" wire from the picture.
Now, I'm not so sure but I don't believe that this is normal (BTW, I haven't mentioned it but the car is almost 10 years old and has 97.000Km. I live very far from the sea (over 1000 km) and other corrosion sources. The air is rather dry although it does rain often). Could this be indeed the cause of my electrical problems? And if so, will spraying an anti-corrosion compound do the trick or will I have to take these wires apart? (hoping it is not the latter because access is VERY difficult and I would probably need assistance from my mechanic).
As for the catch bottle. Although it is hermetically sealed (the cap is the original one) the "max" mark is located at about 3/5 which still leaves 2/5 (or perhaps a bit more) for expansion.