I use DuPont Prep Sol (it's available at auto body supply shops...if they don't carry Dupont, there are other automotive paint companies that make similar solvents.
I put mine in a spray bottle. I spray it on the area to be "de-tarred" and let it sit for a moment...then use a cotton terry towel (one of the old ones no longer suitable for polishing and waxing) to remove the tar. Sometimes you have to make more than one application, escpecially if the tar is "clumpy".
I use clay after the tar is removed to finish the cleaning process.