I don't mean to sound condescending to anyone in my previous post, but as a matter of safety for you it is highly important that you understand what your doing with auto electrics.
Ok, do you have a test lamp or multimeter?
Have you fitted the new relay and fuse in the back?
Is the existing fusebox in good order?
You can test the timer with either a test lamp or multimeter. The engine must be running (so the battery voltage doesn't drop off), and the original fuse removed from the fusebox. Switch on the rear screen demister and probe the live terminal of the fuse holder with the test lamp (one of the fuse terminals will become live when the switch is on).
Keep the probe on and check with a watch / clock that the timer switches off after approx 20 mins, give or take 5 mins.
If it stays on continuously, then you need to source another control unit.
Once your happy it works, fit a new 5A fuse in the fusebox and check the signal now goes to the back of the car, supplying the new relay. If it doesn't, there is a connection either behind the glove box, or under the passenger footwell carpet. This connection can burn out also (on the br/wh wire).
When your happy with that, make sure you fit a 30A fuse fitted in the new fuse holder in the back, and all should be good!