Mercer's understanding with the cars is that the gangers are currently unaware of them - he avoided mentioning them in their presence. Mostly, Mercer wants access to Iki's ride to look for more clues, and if there is any ammo/credsticks/weapons that might be useful he might liberate those, but he's not terribly interested in the vehicles themselves. Ownership might not be too much of a problem as Iki is unlikely to come looking for them any time soon on account of being dead, and an additional vehicle is always handy. I expect that Margot and Tiny will likely come looking for Tiny's ride, so he's keen to leave it for them.
Mercer imagines that the best way to gain access is to start with the jammer so that Iki has limited options for stopping them (if he even cares to do so) and then physically destroying something (a window likely) to create an opening to the vehicles insides. Then disable its wireless and search thoroughly. Optionally, after disabling the wireless, drive to a "safe" location and search there and consider changing ownership. This would require avoiding the law while the wireless is off, but aside from that has the advantage that Iki no longer knows the location of his car and thus can't call in assistance to harass us.
ETA: Margot/Tiny might be more likely to call the car to the church to pick them up rather than try to walk to it in Tiny's condition.
