If you are using a trickle charger that plugs into the cigarette lighter, they only work to keep a charged battery "topped off" while it's parked for extended periods, it will not charge a battery that's low, or keep it charged if you are putting a load on the battery, such as windows, top etc.
How old is your battery? The problem may simply be a bad battery. My experience is that 3-4 years is maximum battery life on my Porsche. Your battery can be tested to see if it can "hold" a proper charge.