Electrical Problems w/ 1999 Golf

I recently have been having problems when starting the engine. The engine doesn't even turn over because there is not enough power. The clock dims out when I turn the key. This happened earlier this summer when I didn't drive the car for about a week and a half. I got the charging system checked but everything came back alright. I have a bass tube and I think that might have something to do with it. I also just had the driver's side door latch replaced because it was causing the alarm system to not work. The car went dead just one day after the door latch was replaced.

If your battery is holding a charge of 12.5 v or more, then could it be a bad ground connection? There are several to check on the Golf, including under the battery tray. All fuses OK, including those on top of the battery (I'm assuming your vehicle is an A4, i.e. a 1999.5 Golf)

Does the issue persist when the bass tube/amp is NOT connected to the electrical system?