kristian, do this test for me, at night, turn on everything. radio, headlights, interior light, etc.
Now watching the interior light, from idle rev the car slowly past idle, do any of the lights become brighter?? and when yo let go of the gas do they dim down again?
Another thing to check is the -ve cable that goes from the -ve terminal on the battery to the body of the car. Its tricky to check, but if you can make sure it is very tight.
Also try to prise back the carpet and under dash trim, on the drivers feet area, when you do this you are looking for Green/yellow cables that got to a bolt head in under the cables and back under side of the fuse box. Make sure this bolt is tight too.
If in doubt see an auto electrician. Paul Vassalo, David Warner autom., or Esscargo/north shore rally sport, are who i recommend. cheers chris