Leaky windscreen, broken electrics

Hi all

I'm on the verge of deciding whether to spend a whole lot of cash on my convertible 1989 vw. Want to keep it, but…

The windscreen has been leaking, and as you may be aware, it's been leaking directly onto the fusebox. This is the state of my electrics now:

Lights and indicators seems to work fine

When lights are off, fan and wipers don't work
When lights are on, fan and wipers don't work
When lights are on, fan and wipers work, if I pull the high-beam leaver and hold it there


Checked fuses and they seem to be fine.  

Anyone have any further thoughts?
I think it may be a fuse-box replacement scenario. Or?

Cheers