My 2000 Mercedes-Benz S430 has a slow drain

My question is lately I have had to emergency jump
start my battery on my 2000 Mercedes-Benz S430
on several occasions and I went to AUTO ZONE to
have them run a diagnostic and the battery is
running at 94% capacity and the Alternator is
running well too.......so it must be a slow electrical
drain leak and I do see a small wire connected to
thw negative post on the battery of which my son
mentioned that his old used LANCER car was set
up the same way and his battery would drain too
because his speakers were wired up to it.  So, my
question is what should I do to go about getting
this slow electrical drain on my vehicle fixed?!  It is
somewhat frustrating and I have had the blessing
of a friends use of her jump start cable box...but,
what if I didn't...I cannot afford to go to the
Mercedes-Benz dealership which they would choke
me for money...I was fortunate to have a free
courtesy call to Mercedes-Benz Roadside
assistance which this very nice guy Moe told me
the first call is free and each additional call would
be $75.00 a shot...so Shaun a Madter Tech told me
my battery was running at 3% capacity and it
would be $175.00 for a new batterywhich I didn't
have...please help me.......so tired over the screw
the stupid woman bit people tend to have!  LOL
Any and all help is greatly appreciated~