@reaver Hope this link works . Basically it's RAI that wards can be anchored to vehicles and move without issue.
@living I need to dig up the link to the discussion on Possession and Rigging for the debate as to if you need a skill to control your own body. Suffice to say if the spirit doesn't know how to drive he can turn control over to the vehicle autopilot, which is fine by me. His contract doesn't necessitate him driving so it's his choice if he wants to.
Sadly, that post is for 4e rules.
(The date of the posts is Feb. 2012. SR5 came out in 2013)
Since then, a host of new mechanics of wards have been introduced. And, while some wards are easy to set up (such as mana barrier) other wards have much more stringent requirements. (Like masking wards - which must be anchored, set up through ritual casting, etc)
Most of these changes were for game mechanic balance as the largest overall complaint of shadowrun 4e was that magic was TOO versitile, causing the game to haveva nickname: MagicRun.
So in 5e, there has been a lot of changes to the way things work. (Direct combat spell damage, wards, drain, etc)
So, in 5e while magic is still very potent, it's not as versitile as it was in 4e, and there is a new range of limitations on just what can be done.
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An other issue you have to remember too is you are setting up yourself for astral intersections with a possessed car.
An astral intersection happens when something with an awakened aura (foci, spirits, all dual natured entities, sustained spells, etc) are forced into contact with a ward.... like you driving your car onto corp property where there is a ward! - best case outcome for you there is that the ward breaks (and instantly alerts the mage that created it). Worst case is that the spirit in your car is disrupted, ending it possession.
However, its not like you are going to gind wards in the middle of the street of public domain. But you COULD find them on Corporate controlled parking garages that are tied to important corporate buildings, on corporate controled/owned lands.
Remember the tagline of 5e. "Everything has a price". For somethings players want, there is always a price or a counter.....
No one is saying you can't have a spirit possess a car. Just realize that you probably can't effectively hide the fact that you have a spirit possessed car from all forms of detection. (Physical, astral, visual, wards)