My car stereo wont turn on and now my car wont

Follow the smaller red positive battery cable to where it's bolted to the under-hood fuse box. Check that connection to be sure it's clean and tight. Check the battery cable connections too.

Why waste time worrying about the radio? You have more serious problems that once fixed will likely fix the radio too. Since the lights are working, I assume at full brightness, that proves my first suggestions aren't the problem, but you should also notice they don't get dim or go out when you try to start the engine. If they DO get dim, you could still have a battery or cable connection problem.

Look for other systems that aren't working so we can see what they have in common. What exactly happens when you try to start it? Do the dash lights turn on and off like normal? Do other switched circuits work, like the heater fan and wipers? Do you hear anything under the hood clicking when you try to start the engine?

If all of this happened at once, you know the radio has nothing to do with the starting system, and the starter circuit has nothing to do with the radio. There has to be something in common that affects both of them. Have you checked fuses under the hood and inside the car?

I was confused earlier but I understand now what happened. GM had a 100 percent failure rate with their previous CD players from the mid '90s, and to insure they got all the repair business for their two grossly over-priced repair centers, they refused to let us buy radio service manuals or parts after the '94 models. In response to $450.00 repair estimates, a lot of people just installed higher-quality aftermarket radios at a much lower cost. Starting with some 2002 models, to combat that and make life even more miserable for their unsuspecting owners, they started building the Body Computers into the radios so you HAD to get it fixed or you wouldn't have power windows, chimes, and cruise control. There is absolutely no reason to do that but we already know General Motors is one of the top three least customer-friendly manufacturers in the world. They make their profits on obscenely high repair costs once they have you hooked with the product.

In response to the engineers' stupidity, the aftermarket manufacturers came up with radio relocation kits that moved the old radio / Body Computer to the trunk. Just the speaker wires were cut to connect to the new radio. I'm sure GM will dream up another strategy to separate owners from their money, and when they do, you can be sure I'll be spreading the word. With all the insane, unnecessary, unreliable, expensive computers that can lock up from simply replacing a bad battery, you can be sure I will never be buying another new car.

I know there are kits now that let steering wheel controls work with aftermarket radios but I don't get involved with any of that stuff. I don't even want to know how they tie into talking to all the other computers on the car because sooner or later someone is going to ask me to fix theirs and it's likely I'll cause more problems. All I work on now is Chrysler radios because some of them are real high quality, I have the service manuals, and I know they'll stay fixed. I'm sure it won't be long though before they copy GM too.

So is there a defect in that interface module or is something just wired wrong? Somehow the original radio needs to be in the car because the Body Computer is the master computer that tells all the other ones to turn on when you turn on the ignition switch. Another trick they pulled is when your mechanic has their scanner connected to the car, there is a drop-down menu with a "lock" selection. If that button is pressed, it electronically locks every computer on the car to that one Body Computer. That programming can not be undone, and you have no way of knowing whether or when it was done, as long as nothing fails. At that point, if the radio / Body Computer fails and has to be replaced, ALL the other computers must be replaced too. Imagine the cost of up to 47 computers, and the labor cost for downloading the software into them over an internet connection. A defective radio can send your car to the salvage yard due to the high cost of repair, and those computers won't work in any other car so they will have no value to the salvage yard. They know that when they buy those cars. Armed with that knowledge, very few of those people are repeat GM customers.

I don't know if the Body Computer is in your original radio already in a '04 Grand Am. If it is not, what is the purpose of that interface module you mentioned? Is it just for the steering wheel controls or does it do something else?