Pontiac Repair: Heater/ fan, light dims, battery cables

Question
I'm not sure what the problem is. 2003 grand prix V-6 3.8
I've read the other questions and answers and it doesn't clear it up for me.
When I turn on the heater 1-4 on the fan switch do not work but 5 does;
what i can't figure out is why the Mpg ,oil life display shuts off when I turn the heater on.
Do you think it's the heater/ AC control head, or the ignition switch, or a relay going bad?

Answer
Hi Matthew:

I was all ready to say this has a bad blower resitor up untill I got to the last part of the question.. I still would say the resistor is bad but you may have a power or ground issue going on here. #1 check the battery cables and makse they are tight and free of corrosion. besides the DIC cutting out does anything else go out like the PRNDLL lamps on the speedo? if so I would possably lean towards a power feed issue from a bad ignition switch or a bad power feed into the switch.. make sure the gounds are tight on the engine and to the body from the (-) cable.. when this dims the lights on the DIC if you have a 12V test light check the power at the blower motor HVAC fuse and see if the light dims down you could have an issue where the blower motor is going bad as well and it has a heavy draw not bad enough to blow the fuse but enough to draw down the voltage on that fuse and possably that fuse and the DIC have the same power feed offf the ignition switch that is why I asked is naything else acts up at the same time as the DIC does.. good luck :)