Plymouth Acclaim 94 Gauges going crazy

HI, I have a 94 Acclaim V6, 3.0, 73000 miles.
My problem is that when I turn on the lights my speedometer, fuel, voltage and water gauges go crazy (This happens at any time with and without engine running).
When I turn on the lights (while car is running ) the the speedometer goes to 0 Mph, the fuel and water gauges goes to high and the voltage goes to low, both turn signals stay on (only in the dashboard), and the high beam indicator is lit even if the car has the low beam, and my dashboard lights go off, but my lights and tail lights are working fine.
They are going over high and low, especially the fuel gauge (it goes much more than the F level).
Everything goes to normal when I turn off the lights.
Something is happening to my tail lights because, when I take out the tail light fuse, everything is working fine, except I don't have tail and dashboard lights.
I tried taking off all the fuses and the tail light fuse is the only one causing problems. I also changed some light bulbs, but nothing.
I noticed something, if I use the battery without engine running, and the voltage is getting lower then when I tunr on the light the gauges are going just a little crazy, but when running and the alternator is charging the battery, when I turn on the lights the gauges are going crazy.
So, I can't drive at night because I'm gonna get a ticket, for not having the lights on, or for speeding because if I have the lights I don't have the speedometer.

Can anyone help me? Because I'm stucked.