How to make a vehicle number visible?
Depending on the fidelity of the video you can try to do a high pass filter on the video, in essence it will sharpen edges of the scene and help bring it out.

The two steps I would do:

1) De-noise it (I use the neat video plugin for after effects)

2) Run a high pass filter (photoshop has this, but not after effects). 
To emulate this:
a) Create a duplicate of the video (command D)
b) Drop the tint effect to turn it into black and white
c) Add the find edges effect (only the black areas will be sharpened) 
d) Drop the levels effect onto it, adjust it to to bring in more black (to sharpened more) or bring in more white, to leave out areas not to sharpen.
e) If the edges are to harsh you can always blur it to shoften or feather the sharpening effect.
f) Add an adjustment layer, drop it below the duplicate layer
g) Change mode track matte of the adjustment layer to luma inverted (it would hide the layer above when this is done automatically).
h) Add unsharpen mask effect to the adjustment layer, put it 100, radius 2, adjust as needed.

You can already see that this type of work you can tweak a lot of things.
Hopefully this gets you going in the right direction

All the best,