You need to do some diagnosis with a voltmeter.
o It could be that the alternator is not producing enough juice. If you notice your lights going bright and dim with revs then that would be a certain sign. Check this by measuring the output voltage at fast idle (1450+) with all major electrical loads (headlights, fan, seat heaters, window defrogger) turned off. Checking the output at the alternator is preferred, but at the battery should be just fine provided the primary voltage wiring and engine/chassis ground are okay. You should see at least 13.5 volts (should be more like 14.25 volts). If not, this will normally indicate a bad regulator and/or worn regulator brushes. The regulator pack is in the back of the alternator and can be easily replaced. It can also indicate that one or more of the internal diodes in the alternator has fried, especially if the car has been recently jumpered off another vehicle and it was done incorrectly. Bad diodes used to be a common problem, but not so much any more. A constantly slight glowing of the charging dash light is often an indication of bad diodes.
o It could mean a bad battery. If your battery is anything over 5-7 years old then that is very likely. For an old battery it may simply lose its charge overnight. The idiot light will either not glow at all or will glow solid bright if the battery plates collapse internally and short out a cell. The simplest way to check this is to fully charge it disconnected from the vehicle then reconnect it and see if it can produce and hold a full 11.5-12.25 volts under a moderate electrical load (like fan on high plus park lights) with the ignition off. Check it again in about 8 hours if you're suspicious, but disconnect the battery in between in case there's a constant electrical drain you aren't aware of.

At this time of year, I'm betting on your battery. Check the 700/900 FAQ feature section here on this site for more tips on charging system problems and how to deal with them.
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Dave -not to be confused with a real expert, just goofing around at this