99' Grand Am SE - Starting Problem / Power Issue

Hey everyone. Looken for some help here. Just started 2 days ago. Went to start the car, it would sort of start, then turn right back off again, but all the lights would stay on the dash. So I clicked my key back to the "off" and restarted it. Started just fine. But everytime I start my car now, it does it. Its not like it having any problem turning over, because it works the 2nd time perfect. Now, with this, the security light was on the last time I drove it. It turned off though after I restared the car. Also at the same time, my XM radio unit will now just turn off. The main radio does not. I have a external deck wired to my fues box. Also, when I hit my bracks, my lights seem to dim. And when I turn on my blinker, my lights seem to turn on/off very fast. Like a split sec. speed. She has been running perfect up to this time. When I drive it, I feel no power lose at all. Anything?

Thanks,

Chad

Did all of this start with the addition of the external deck?