You can only open carry in a vehicle with a permit, which you have. However, you live in Philly, that is not a place to have a visible weapon permit or not. If you get stopped by Philly PD they might be taking your gun, here is Ortiz vs Commonwealth on the topic of carrying in Philadelphia
http://www.cs.cmu.edu/afs/cs/user/wb...mmonwealth.txt
I know your rig wont work with your seat but don't mount it to the side of your trans tunnel, yes technically you are allowed to do so but its not saying you wont have to show up to a courthouse sometime to prove that point.
As for getting stopped in PA you are not required to inform law enforcements officers that you are carrying a firearm, but if I get you out of the car for something mentioning it to me will make me a happy camper, better that then me seeing it. Just turn your interior light on and sit with your hands on the steering wheel and wait for the officer, if you tell him you have a weapon in the vehicle he might remove it for the remainder of the traffic stop or choose to not do anything its up to them.