Find Morningstar after fully claiming their Territory

Choosing to give Josh over to Cyrus results in you receiving a "City Takeover" bonus, separate from the City Takeover upgrade. Choosing this bonus will not lock out the regular City Takeover upgrade.

Using this bonus allows you to take control of one large area of the city of your choice. This area is outlined by a bold purple line, includes more than just a single Diversion/Property, and is named, like Loren Square or Sunset Park (basically, the neighborhood division lines).

It does not have to be within Morningstar territory. Taking control of the area will automatically complete all Diversions in the area, as well as automatically give you all properties in the area.

As soon as you customize a car, it becomes permanently yours. If it is lost, stolen, destroyed, flung off a cliff, etc, it will return to your garage a short time later.

Sometimes you can't access the shop's garage (especially if you have a wanted level) so you might want to hide someplace you own for a few seconds to reset everything. The garages at your hideouts always work though.

For reference, you can also customize cars at your hideout garages, so there's really very little use for the car shops themselves, besides being a place to hide and a sometimes convenient place to store/fetch cars.