Battery drain will not start

My vehicle listed above was driven last on Monday, then I went to start it up this following Tuesday but got nothing not even a click or anything but would continue to ding once the keys were out (which ended up having to disconnect the terminals per mechanic suggestion). It has done this previously, but we were able to wiggle the connectors to the batter and tighten them back up and it would start unless the battery needed to be charged. Brand new battery not even a year old. Alternator is about 3 years old. I was wondering if anybody has any idea where I should start to look for the problem and get it fixed? My mechanic doesn't know where to start either and I don't have the money to just keep replacing parts. Only aftermarket part is a stereo.
It has also randomly been having the ABS, battery and 1 or 2 other lights pop on but go back off and the gas gauge and temperature gauge won't work while lights are on.