Use this formula for the source's x slider:
xtopercent(counter(0, grid.lastx, pulse(1), 0, grid.onactivated))
The source will go forward one step at each second. Modify the pulse(1) if you want it to go faster/slower.
If you don't want the source to start at position 0 each time, you can specify an offset like this:
xtopercent((counter(0, grid.lastx, pulse(1), 0, grid.onactivated) + 5) % (grid.lastx + 1))
where 5 is the starting position.
Cheers,
Mathieu