Flat battery on GTI 16valver

[RKett 16v]

Hey guys im a new member although i'm not that new to the scene. The problem is that my 1990 gti's battery keeks goin flat. The car is in storage  at the moment untill April and has been since October (Couldnt refuse it at the time but need to finish uni first!!). I've got a brand new yellow top battery due to the planned install. When i brought the car it has two carpeted blanking boards in the boot across the strut towers of which the left houses a 6 disk changer which i think is powered off the old boot light (The car was once an Audio images car i think) which i think is the source of the problem, does it need an earth? Any suggestions guys thanks in advance.

[Blue MK2]

the changer is powered by the headunit though right

Could be the actual boot switch that keeps the boot light on, causing the batt to go flat

[RKett 16v]

Yeah its powered to the headunit. The pressure switch on the left hand side. Not sure but think these may be wired together so the changer doesnt turn off when the boots shut. Any ideas on how to wire it up so that this isnt live all the time which is presume is occuring at the moment.