Here's how to play it.
The Wagon is a 3 by 5 rectangle. Yes, 15 by 25 feet is a little extreme for a wagon, but otherwise we wouldn't have enough room to fight. The row by the horses is considered difficult terrain, as there is a seat for the driver there.
The horses need to be modified a little bit too. There are two horses to this wagon, but having two large creatures isn't really going to cut it in terms of spacing or realism. Use the horse statistics, but make them one square by 2 squares. Sacrilege, I know, but it makes far more sense conceptually.
Throughout the encounter, the wagon is traveling at 40 mph, or about 360 feet per round. Obviously, just draw the wagon on the battle grid. The cliff is 1,800 feet away from the start, or 5 rounds.
The fight proceeds much like any other fight does, but in much smaller quarters. Casters suffer a concentration check for vigorous motion (10 + spell level), but other than that things are normal, if a bit cramped.
At the end of the Second Round and the end of the Fourth Round, the wagon will hit a bump. All those on board must make an Acrobatics 15 check or fall prone.
Stopping The Wagon
Players adjacent a horse can make Handle Animal 20 checks to calm it. Both horses must be calmed to have any effect. If both check are made before the fifth turn, then the horses stop before the cliff. If both checks are made on the fifth turn, then it is too late to slow down and the wagon will still plummet over the edge.
If either horse dies, or the wagon is dealt some cataclysmic or unbalancing blow, the wagon immediately crashes. If the wagon crashes, all players are thrown from the wagon, and are dealt the appropriate damage. They are dealt an additional 3d6+10 damage (Reflex 20 halves) from the wagon splintering apart.
The horses are connected to the wagon by a thick wooden pole, or "tongue," that runs between them. If this can be severed before the fourth turn, then the wagon will stop before the cliff. If the tongue is severed on the fourth or fifth turn, then it is too late and the wagon will not slow down in time. Because of the vigorous motion of the wagon, the tongue is treated as a wooden object with Armor Class 14, Hardness 5, 20 hit points, and a break DC of 23.
Falling off the Wagon
Each round the wagon moves 360 feet, so any character that falls off the wagon is effectively out of the combat. However, if multiple characters fall off the wagon, it is possible for them to engage in combat, given that they fall off the wagon on the same round. Draw a separate battle grid for them to engage.
Falling off the Cliff
Falling off the Cliff is a really bad option. You should always warn the characters on the 5th round that they either should jump off the wagon now, or they are going off the cliff. The cliff is 100 feet tall, but the added forward momentum of the wagon yields damage as if the characters had fallen 110 feet. Additionally, the wagon splinters and breaks upon impact, dealing 3d6+10 (Reflex 20 halves) damage to anybody who remained on it. Overall, this can amount to 14d6+10 damage: on average, 60 damage.