Sometimes with combo units, you have to physically switch from dvd to video and vise versa, simply inserting the tape and playing possibly stiill leaves the unit in dvd mode , hence the no signal.
It may be as simple as switching from one to the other with the remote.
Another possible option, Is if the ariel is also looped through the Video and then to the TV, if so with the video playing, you should see the counter operating , pause it, then scan the analoque channels on the tv and see if it finds the still picture, then note the channel.
It may be the lead is switching the TV to the scart input and looking to see a dvd playing.
Though I would have thought both DVD and video play via the same scart.
I may be wrong, but worth a try, but without the relevant manuels its hard to say.