It sounds to me like a short or voltage drop somewhere in the wiring or connections.
Do tests for voltage drop on your wires and also the connections. Test between the battery terminal and the wire going to the battery, for example. That should help you pinpoint a faulty connection.
Google for "how to test for voltage drop" and it should give you an idea of how to go about it. I have not read this article myself, but I love the articles on their site... this should help you... http://www.aa1car.com/library/voltage_drop_testing.htm
There's also a really good book called "How to diagnose and repair automotive electrical systems" which I highly recommend.