[R@S]

You need to drop off your car at the bartender before you'll get the dialog option to leave.

EDIT:

You could do me a favor by checking the exact syntax of the dialog option you get when having a car and not being able to continue the mission. If you do I can add the "first leave your car" in the dialog. I found two very similar entries in the e6q.txt and am not sure which one to edit.

It's either 1:
"Wait a second, I just need to do this thing here..."

Or 2:
"Wait a minute, I need to take care of one thing here..."

Or it might be something else.