Why do you need a timed relay to do that?
Considering the top down operation ... if you wire across the contacts of the existing switch with a simple toggle switch (one that isn't spring loaded to off/centre) the top's operation will complete (assuming everything is working correctly and there is enough hydraulic oil) and then stop.
The same would be true of putting the top up again.
So, to my simple mind, all you need is a centre off rocker or toggle.
Maybe one of these on ebay will do the job:
http://bit.ly/JlAZdARegards,
Mike
Notes: I haven't looked at the wiring and don't know how much current goes through the switch. You'd also have to manually put the switch to centre at some point.