Where is voltage regulator in 1976 El Darado Cadillac?

If I'm not mistaken, GM has had the voltage regulator internal to the alternator for years before '93.
The alternator has to be disassembled to repair the voltage regulator. Most do-it-yourselfers just replace the alternator.
Here's a picture that shows the regulator at RockAuto -

1993 CADILLAC ELDORADO 4 9L V8 Voltage Regulator RockAuto

The dimmer switch for the headlights should be connected to the turn signal lever, you pull it towards the steering wheel to change.
The brake light problem could be a short in one of the bulb sockets in the back.

it is integral to the alternator. it is inside the alternator

where is the fuse box on a 1976 cadillac eldorado

A 76, huh, let's see, I don't know if that had the regulator in there or not. It sounds like the alt died, but maybe not. It won't run without the alt. And will only last about 20 mins on a full battery. I would take the alt off and get it back to the store to have it checked. That would be the quickest way to tell, just because it is new doesn't mean it is good. Don't keep on killing the battery, it can only handle so many drains. Hope this helps.

Generic code is Cylinder 1 misfire

sounds like alternator needs replacing, could have bad diodes,or regulator is going bad,

Diagnostic trouble codes.

http://myweb.accessus.net/~090/dtcobd2.html

Likely you have one burnt diode in the alternator...If so, you will have marginal charging rate when system is not loaded and below normal rate with items such as a/c, fan or headlamps on high setting. You can check this by putting a volt meter across the battery terminals and watch the voltage. With no accessories on you should read around 13-14v. With all accessories on volts should not drop below 12.9, nor should it continue do drop over several minutes. If it does, you need to either replace diodes or replace alternator.

try factoryonlyparts.com also try searchhing online for car clubs that can help you find parts.