Open up the rules file, which you can extract using
XCC Mixer.
Search for the unit you want to enable, say it's the Police Car, you should find something like this with all the unit information listed below it;
; Police Car
[COP]
UIName=Name:PIGCAR
Name=Police Car
Now we edit out the bits we need, and fix them so the car is buildable.
; Police Car
[COP]
UIName=Name:PIGCAR
Name=Police Car
Prerequisite=NAWEAP*
Strength=100**
TechLevel=2
Speed=8**
Owner=Russians,Confederation,Africans,Arabs,YuriCo untry**
*It's probably best you leave this as it is, unless you want the car buildable from a War Factory after a Lab is built.
**You can edit these to increase the strength of the unit, make it faster, or change who can build it, respectivly.
I suggest making one other change; disabling the siren of the police car. Do this by changing: AmbientSound=PoliceSirenLoop, to: AmbientSound=Dummy
Oh, and hey Ruben.