I have a 02 sihlouette and I am having troubles

I have a 02 sihlouette and I am having troubles. I had run low on brake fluid, so I put some in. Immediatly my low brake fluid light came on, as did my door open light and rear hatch ajar light. I took to a local machanic to see what was goin on he tried to hook up his computer and would not read because the voltage was fluctuating at a really low rate. He said that I would probably have to take it to gm. I was woundering if you had any ideas. Thanks