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When you do this it will actually be less of a "load" on your battery. Old wiring tends to add "load" on the circuitry.....which you don't want. Adding a load is like adding more lights/motors. With the relays you use the old wiring to control the relay....which switches direct battery power to the light/wiper etc. Just make sure your wiring to the light/wiper is the correct size.
to add to the above, by eleminating old wiring, going directly to your lights, door locks, windows, etc utilizing a relay.....your decreasing the load because they are "directly" connected to the battery....eliminating old wiring, connectors, etc. Your original wiring will only have to control a pretty small load (the relay).
In the forums and other people say that if you do this for the headlights for example, you get a much brighter headlight beam, and more consistent. On the windows, it says windows will go up and down faster. I am looking into this because the previous owner has done some bad hacking where one of the windows seems to be powered through the battery but also somehow connected to my headlights? So if I'm going in to start taking stuff apart I might as well replace the wiring. LMC sells the headlight harness for $22 and the window one for $44 (both windows).
The old wiring and switch configurations add a ton of resistance to the circuit and are damaging to the switches. When you convert to using a relay with power coming through a larger gauge wire directly from the battery, you remove resistance(that converts voltage to heat) and allow the windows to operate faster with less draw from the battery. It also prevents the large current arcing the contacts on the window switch and wearing it out prematurely. It is a much more reliable and efficient way to manage the electrical system.
