1995 ES300, 130,000 miles:

I've noticed that when I've got my headlights on and I press the brake pedal, my headlights dim. The same thing happens when I use the window controls. Once in awhile, the airflow out of the vents slows down somewhat under the same conditions (the airflow part might be shoddy memory on my part, but I'm pretty sure that I recall that happening).

Is this a battery or alternator issue, something else, or just to be expected?

 

I've owned my 94 ES for almost 13 years now, and it is on its fourth battery.

With each different battery, the headlights have dimmed when the brakes are applied.

I have checked the voltage numerous times when the car is running at various RPM's, and it has always been around 14.4 volts. Obviously, there is no problem with the alternator.

I think this is a common condition in these cars.

 

This behavior is normal with any car, however you can reduce the dimming by cleaning all of your grounds. There is some kind of grounding kit available for the ES (do a search). There is really nothing else you can do.. But make sure grounds are good as Toyotas typically have corrosion problems... My rear defogger really sucks the amps...