Dynamically changing a decals position on a mesh surface

I am experimenting with the builtin decal shader which I have applied to a vehicle model. I would like a user to be able to control the position of the decal at runtime using arrow keys for example. So after a user selects a decal texture that they like, they can then smoothly move it along the surface of the vehicle and choose whether it is positioned on the bonnet, the quarter panel, roof and so on. I have tried adjusting the uv position in the material settings but whether the texture is set to repeat or clamp, this either makes the decal get repeated on the opposite side of the vehicle or simply stretches the decal. Here is a game example of what I need: