I have a 2001 Grand Prix GT. I got a aftermarket deck installed in my car (for the distracted driving laws) and now my car dies after it sits for a day or two. I took my car to a local parts store and they did a battery and alternator test and said that they looked good.Can anyone tell me what this could be caused by?

Tumb of rule for starting up a car> spark, fuel, and compression.

Just start investing the causes of failure to all of them

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Why have more money than brains? Cuz RACE CAR :

It starts fine, it's something draining the battery (i think). But when the battery has the juice to turn over then engine it starts easily.

Was the battery charged when you brought it to the autoparts place?

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Why have more money than brains? Cuz RACE CAR :

Parasitic draw test it. Get a multimeter, set it to mA, disconnect the negative battery terminal and connect the - side of the meter to the negative cable you removed and connect the positive side to the negative battery terminal, then see what the amp draw is. If it's high, start pulling fuses 1 by 2 until that draw drops to about 50mA. The fuse you pull which causes a drop is the circuit causing the excessive power draw when the key is off.
Google parasitic draw test and you'll get a better idea of what to do since my description sucks. Also go to Eric the Car Guy's youtube channel. He has a really good video on how to perform the test.

Edit: I went and found the video just for reference. The GP's have 2 fuse boxes, 1 in the engine bay on the passenger side strut tower, and one in the glove box.

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Dubs:00 VW GTI VR6, Low and Slow 91 VW GTI 8v, Shave the Bay!! 91 VW Cabriolet 8v, bitch Basket
GP's:04 GP GT, Past now G-Ma's Girlfriend's 97 GP GT, "Project Pink Power"