One thing that you can be sure will happen while gluing vinyl or leather material on panels, seats, etc., is that you will get some little splotches of contact cement on the "good" side. All attempts at preventing this are to no avail, because when you step back to admire your finished work the spot will show itself.
Before you get out the 409 or Mr. Clean and the scrub brush - or worse, some solvent - to attempt to remove the glue, stop! Sometimes these methods will remove glue, but they tend to slightly discolor the vinyl or leather. Don't bother with these.
Instead, grab your roll of vinyl (or aluminum) duct tape and wrap a little bundle around your fingers with the adhesive side out. Pat the glue spots with this bundle and watch how quickly it comes off. This method even removes glue from sewed seams, something that is nearly impossible to do with cleaners.