Haven't a clue how it all started, tho we at the time were having a problem with what we call roof rats (probably Norwegian Greys).
I did notice droppings under the car in garage, but didn't make the connection until we got to smelling them when the AC was on.
We drove up to Canada this past summer, camping along the way. Perhaps (tho I doubt it)they got in at that time.
I think, because I was driving only 3 times a week, the buggers had an opportunity to begin building a nest. Apparently, no animals were found inside the car.
Needless to say, I've turned my garage into a death trap, spreading packets of warfarin all around/under where the car sits.

Invested in this case is used as a military term, tho extended somewhat.