1991 525i not starting (battery draining problem?)

Hi, new here, we got the car a couple days ago. It was running fine then, although suspiciously the guy we bought it from had it running when we got there, leading me to believe he knew about the problem already and didn't want us to. The problem is that after my dad drove it to work yesterday, he tried to start it up at lunch and it wouldn't start. It didn't sound like the starter was trying at all, just a bunch of electronic clicks. However, the electronics all work, dash lights, headlights, locks, etc. It just doesn't seem to be able to start up.

When we took it to a shop for an unrelated issue, a mechanic said there were wires hanging down in the trunk for a sub, and we knew this but thought it was no big deal. He told us that if it was wired wrong, it could be draining the battery, even without being hooked up to a sub. I checked the battery, and there were a few extra wires hanging off the negative terminal. However, that doesn't make sense to me, how could a couple of wires attached to just the negative terminal do anything? Anyone know what's really wrong?

Have a Load Test done on both your Alternator + battery.

The Security system, Computer, control modules, electric fuel pump, and all those bells + whistles can demand alot of power.

How old is this battery? , it might not be holding a charged like it once did!

It's a Duralast that doesn't look very old, certainly not as old as the rest of the car. Also, I checked again, and the wires were hooked up to the positive terminal. Hm.