Check earth from battery to chassis and to gearbox. If the connections are dirty/corroded you will find funny things happening which you wount usually be able to predict. Turn everything on and drive again, if heater and wipers slow down i should think this is the problem. If these are fine, then the problems lies in the stereo and lighting circuits. Unlikely.
Also check link +ve between battery, starter and alternator. Again, corrosion will play havack with electrics, clean and grease connections, should be better then.