I have a 75 1.8. I just got this car about a month ago, and added a 300 Watt amplifir and a single 8" sub-woofer to the already upgraded stereo. Then, about 2 weeks later, my alternator died. I believe that this untimely death was caused by the additional draw on a 31 yr old alternator. I have replaced the alternator with a rebuilt stock Bosch unit and a new regulator. The system seems to be charging fine now. My question is this, How much more draw wil the system handle? I would like to upgrade to the halogen Headlight system, and add seat heater elements into the seats when I have them redone, (soon I hope, but delayed now due to the alternator expense!). I don't think that the headlight upgrade will increase draw much? But I am concerned with the draw from the heaters. I also realize that whenever I use the seat heaters here in So Cal, I will probably also be using the headlights, and I would hate to be in an either/or situation. Also, this car has carbs, so I think that I am saving some current there.

Stock alternator is 50 Amps, so 700 watts tops. No alternator likes to run at over 80% capacity for a long time, say over an hour at a time. That puts the normal max load at 560 watts. I'd say a 300 watt stereo puts you way over the design load.

i had a 300 watt stereo and 4 big H4 ralley lights on my super beetle.
you could actually feel the car slowing down when you turned both on at the same time.

that can't be good, in the long run ..
Andy

FYI - Just because the manufacturer of your amp calls it a 300w does not mean you are going to get that out of it. Most amplifiers are rated at theoretical output. Look up the specs to see what the actual current draw is. Also if you need to save some watts look at using a switching amplifier and speakers with a higher input sensitivity. 3dB of input sensitivity will save you half the power (in theory).