That's the problem right there so.
The Mohair roofs have to be treated two or three times per year or the water starts to drip down inside on the seats etc.
I use '
Autoglym Convertible Soft Top Clean & Protect Complete Kit'
http://www.autoglymprofessional.com/en/products/convertible-soft-top-clean--protect-complete-kitIt's a two-part process and it's very fast to do.
The first is a spray to clean the material, it comes with a special sponge which works great on mohair and does not damage the surface.
Rinse off, then spray on the sealant and just let it dry. When it's dry the material looks darker, like new etc and the water beads on the surface like mercury drops.