One common but often overlooked problem with earlier 911 convertibles (or cabriolets) are bad drains.
If you have a cab, just go and check it now - it'll take a minute! Feel the back carpet behind the front seats - if it's dry, great! Now, lift up the rear seat cushions (mine were only held in by velcro) and feel again. If they're dry great again!
But if either one are wet, then you've got some problems to fix. Luckily in my case it was very easy, and very cheap.
First off, tilt your front seats forwards and remove the rear speaker covers - it's only 2 torx screws.
- wet carpets, seats and then mould, stale smells etc
- damage to the electronics located under the front seats (stupid place to put them, but hey europeans, amirite?