the most simple and effective way to record a ghost for an opening door for example is:
- open your track and place the car on the block where the door is supposed to stand while it is closed
- stand there for a few seconds until the car doesn't move anymore
- press "s" to save a replay
- drive into the direction in which the door is supposed to open
- press "s" again to save that replay
- import the closed door replay into a group in ingame mt
- set start offset to four seconds (the car won't move then)
- set the replay to endless play (it will stay "closed" until the next trigger is activated)
- import the opening door replay into a new group in ingame mt
- set start offset to whatever makes the car move (gotta try a bit until you find the correct offset)
- set it to endless play (it will stay "open" until the next trigger is activated)
this way, it doesn't matter how much scenery your track already has. replays will only last some seconds, so the filesize is minimal. you can easily use a hundred of those replays without making the track too big for online play (i used 143 ghosts in a dragons' tale for example). you don't have to rebuilt parts of the track and your car will be positioned exactly where the "door" needs to be placed (which gets pretty impossible if you start from somewhere else on the track).
edit: uploaded the skin of a StadiumCircuitBase:
http://www.trackmania-rpg.com/index.php?page=DownloadDBData&dataID=30