This is a very good question that you pose.

Car electrical systems are limited in the total amount of power delivery that they can output at a given moment. Large power amplifiers like the ones you've mentioned can chew up more power than what the car is capable of producing when a big note comes, which is why the headlights on the first car dims. The second car may have more problems, since merely turning on the amplifier shouldn't cause all of the electrical equipment in the car to turn off, but i suppose it is possible.

Given the monstrous amplifiers they're installing, they'll want to add a capacitor to the 12-volt electrical system to decrease the load the amplifier puts on the car's electical system. These large capcitors store a good deal of power when the amp is using less power during soft passages, so that when the big one comes, the amp consumes the current from the capcitor before feeding off the car's electrical system. This works because bass passages in music are not constant, but rhythmic. The capicitor charges in the period between booms. Here's a link to see some examples from Amazon.

The capicitor will most definately take care of the dimming on the first car, but the second will need more investigation. It could use a new car battery, but there could be a short somewhere between the battery and the amp, or it may not be properly grounded. If you purchase a capacitor for the first car, try installing it in the second car first. See if that helps solve the problem. You'll undoubtedly need a capacitor for the second car anyway, but beware that it may not entirely solve the problem.

Whatever capacitor is chosen needs to be capable of supporting a system to about a 1000 watts, which should allow a little overhead to be safe. Post back here on their progress to help others figure out car audio issues like this one.

Mind Over Matt'er - Technology musings, opinion, and more straight from TechLore's head geek.

Is your amp an 800 x2 or an 800 bridged? I ask because adding a second sub is still placing a similar amount of strain as it would with 1 sub bridged.

I would replace the section of 8 guage wire, it'll do nothing but give you headaches at this level of power. I'm not going to say that it is the culprit for the amp shutting down (maybe...), but it's like driving a race car with 14" tires from a Ford Focus.

What kind of car is it? Include year too. What size fuse was installed after battery. Somewhere you have something that is not connected right.

Also you mentioned one of the amps getting warm, They normally get pretty warm, if its a good amp it should have a thermal shut off switch. Check the ohms of the speaker and the lowest implements of the amp. How long is the 8 guage wire? If its less then 4 feet I think your ok.

Amazon seems to be having some problems at the moment, which is why the link isn't working. As soon as Amazon gets them resolved, I'll make sure that the link works.

It looks like the PC10F will be fine for the vehicle that is having problems with the headlights dimming, but I can't make a recommendation on the car with more serious issues until they are resolved.

Have you had any luck in the diagnostic yet? Paul is heading in a direction that also merits investigation.

Mind Over Matt'er - Technology musings, opinion, and more straight from TechLore's head geek.

I think you miss read my post. With the amp size you said I would not want more then 4 feet of 8 gauge wire. You are pulling so much current, that with the smaller wire the resistence goes up. This will cause the voltage across the +12 and -Bat to drop, and inturn cause the amount of watts (Power) the amp can draw. Watts = Volts * Amps. But with only 1 1/2 feet you don't have anything to worry about.

The "Thing" you talk about seems to be a distrobution block. Do you have a voltmeter?

No changing the wire wouldn't do anything. By voltmeter I ment a tool. If you want to fix this yourself you will need at least a multimeter, you can pick them up at Radio Shack for $20. Just get the cheapest Digital one they have.