BMW318i Electric window error

Hi I have a 1991, E30, 4 dr, the windows were all working fine - but just recently they have all stopped working unless one of the front doors is open - in which case they all work fine ? - strange, can anyone help with this please I bought my 1993 bmw 325i at a tow yard and the windows do not work. i know a bit about cars but was hoping that some one else that has had that same problem could help me and point me in the right way