Check out the second paragraph of the Ranges section in the ArM5 corebook, p.111:
"A spell that has a continuing effect remains in effect even if the caster moves out of range. A spell that allows the caster to control the effect only permits that control as long as the caster is within range. However, it does not expire if the caster moves out of range, and he may control it again if he moves back into range."
Keep in mind that, in the case of a magic item, the item is the caster.
So, in your case, whether the grog touches the cart is irrelevant. What matters is whether the wand touches the cart. As soon the wand stops touching the cart, the cart's movement goes into an uncontrolled "default mode" (which depends on how the effect is designed -- the safest options are that the cart hovers still or that it gently loses altitude until reaching the ground).
Control of the wand is a different issue. A magic item is not activated by thought alone, unless some magical effect to read the user's thoughts is instilled into it. If that's the case, the grog can control the cart without ever touching it - the wand must touch the cart, and the grog must be within range of the wand's mind-reading effect. Without a mind reading effect, the wand is controlled by some physical activity that a normal human could perceive (e.g. vocal commands or gestures). Yes, you can make the wand operate at a distance, levitating the cart whenever it "hears" a shouted "up". But that has obvious dangers, particularly if the wand has been created without user restrictions (see ArM5 p.99).
However, there may be a way to achieve what you want (which seems to be allowing a grog a Moon's worth of a flying cart, without giving him the wand), with an effect I'd name "Carrot for the wooden mule". Inscribe a rune onto some point on the cart, and a similar rune onto some non-magical object in possession of the grog - say, the tip of a staff. This is a non-magical process. When instilling the effect into the magical wand, make the Rego Herbam effect "the cart('s rune) follows the staff('s rune), with a speed proportional to the distance". Then, a grog sitting on the cart controls the cart with the tip of his non-magical staff very much like the stereotype of the mule's rider who dangles a carrot in front of his mount. Note that the magical effect requires no active control by the caster, so the magical wand need not touch the cart after casting. If you want to be a real munchkin, make the effect with Target:Circle and Duration:Ring and use your magic item to scribe a large enough Ring on the cart (the cart must be sturdy enough that, if a giant picked it up by the portion within the Ring, the cart would not fall apart - most good quality carts would qualify); this reduces by one magnitude the effect compared to T:Ind and D:Moon, makes duration effectively unlimited (until the Ring is broken), and can be used to fly stuff much larger than a Base Individual worth of Herbam.