in theory (wich is not alwayse correct) when your battery starts to die it sulphates wich increases it's internal resistance. This should increase the current flow from the altinator to the battery but if the battery cannot accept the extra current then it will dicipate thru the cars circuit hence giving the faster wiper speed.
since P=V/I, P being power, V being voltage and I being current. The "power" of your altinator is constant so as the current increases the voltage must drop.
put a multimeter across your battery and see if the voltage drops. if it does you need a new battery. the voltage drop also explains ya sub dropping out.
that's way too bloody technical for this time of morning.
hope it helps. If I'm right 6 pack of bundy's to me
