How are wires wired together and where can I find that point?
EDIT: Short answer: You, my friend, need a qualified electrician.



Building code in US and Canada requires that no connections/junctions can be hidden (watch Mike Holmes...), they must all be accessible, i.e. in a box with a cover plate you can take off.

Unless you are on Mars, seeing 30V is bad, and seeing 2V is also bad.  Do you have your multimeter set to read AC???  I always make that mistake of having it on DC.  You may have it on DC and be seeing some other bias voltage (that shouldn't be there either...).

If you are really seeing 30V AC, and you are in the US/Canada/Western Europe, then there's a problem with your wiring.  The fact that turning a light on causes a large voltage drop may mean that somebody wired your circuit in series rather than parallel.