I changed the water pump on my dad's 2005 Impala LS 3.8L and now the blower motor and interior lights don't work at all. The radio plays but does not light and no other lights work either. I am terrible at electronics please help!

 

I would check every electrical connection under the hood. You may have blown a fuse or three, or it may be something else like wiring not plugged in. Too, these days, vehicles often come with two "fuse boxes". One inside the drivers door usually, and one mounted in the car's engine bay. I'd check all those out first, were I you.

These transverse motor, FWD vehicles are not as simple anymore, like they used to be when RWD vehicles were around. Not many of those coming off the assembly line these days, unless its a pickup or a muscle car, like a Camaro or a Vette'.

 

I have checked all the fuses and they are good I checked the voltage at the blower motor with the fan switch set on high i'm only getting like 5 volts. My thoughts are maybe a bad ground? Everything worked fine before the water pump change. I'm not sure what the blower and interior lights have in common? Thanks!

 

The blower motor & controller have 3 fuses that need checked....
Under the hood >>>> HVAC BLOWER
Left I/P >>>> A/C CRUISE
Right I/P >>>> DIC/RKE (Interior lights too)
If these fuses are OK , then check to see which fuse does not have 12V when the key is in the ON position.