What's wrong with my car?

I have a 2002 Grand Prix that I recently purchased with 130K miles. It has the following issues some of which are related, others which are not: P0420 Catalytic converter code, Heat only works on setting 5, lights flicker when going through the different levels of heat/AC, rear defroster doesn't work, lights behind fuel gauge and in the radio goes out at times. I have done the following things: Checked the fuse box for the blower motor which does show the proper voltage. The resistor had already been changed prior to my purchase (I was not aware of this), I changed it with a brand new part again which made no difference. So it apparently is not the ignition switch, blower motor or resistor. The control module? Random water damage shorting parts out? There are no obvious signs of water damage.