I think Daniel was looking for a one-click-solution, cooldude
Quick answer - there is none. When blurring the shadow pass, it blurs all edges of the shadow-layer, so also around the tires/car, which will make a white space in between. There is no way for the program to know which side of the shadow to blur.
The easiest solution would be to render the shadow as soft in Anim8or. Takes a little bit longer but it will save you time and work in post production. Plus this will give a more accurate result (less blur close to the object, moreso far away)
An other solution would be to use the blur tool in Photoshop or what other program you use and go over the sides of the shadow itself. This might save you time rendering but is very labour intensive in Post production.
Good luck trying to figure that out!