What you need:
Dry ice: 20-50 lbs. While you can do the job with 10-20 lbs, the more you have the quicker it'll go. It's worth the extra money. I used 40 lbs.
Hammer: ball peen is the way to go. A heavy dead blow hammer was also particularly effective.
Pillow case/nylon drawstring bag: 1 for every ten lbs
Thick gloves: dry ice will give you frostbite and a chemical burn if you handle it with unprotected skin. Be careful and don't let it ruin your day.
Step 1: use your razor knife to score lines in the tar. Make sure to press hard as to cut as far thru the tar as possible - this makes life easier later. Cut lines horizontal then vertical across creating squares. The smaller the better - but still a decent size piece maybe 3x3 or so. Go ahead and cut all the tar in your car now and be done with it.
Step 2: take 10 lbs or so of your dry ice and put it in your bag and seal it up. Use your hammer to break it into small bits - think the size of refrigerator ice cubes or a little bigger.
You can use the solid slabs of ice, but they aren't as effective on curved areas like trans tunnel.
Step 3: place your bag on the tar. Work dry ice bits out evenly over the area.
Step 4: Let sit for 25-30 minutes. Use this time to walk the dog, enjoy The weather, work on another part of your project, or something else that's awesome. PRO TIP: nestle a few bottles of your favorite beverage on the ice and come back to ice cold goodness.
Step 5: STOP! ... HAMMER TIME! Time to hammer the crap outta the frozen tar. I found that if you give the tar lots of little taps all across the tar it helps to free the tar and leave you less clean up later. The give some good whacks and the sections will start to pop up leaving clean painted metal. Use your shop vac to clean up the small shards/dust left behind. If you run into a piece on the fringe of your area covered by your bag that doesn't willingly pop up you can scrape under it to free it - but I'd suggest re freezing it and then hammering to leave less clean up. ***Don't forget to wear your safety glasses. I don't know about you but frozen tar in my eyes doesn't sound like fun. ***
Step 6: once all the big pieces are up use your razor scraper to remove what's left over. I've heard WD-40 helps at this stage but didn't try it myself. Of course, if the colder the better.
Voila! You're done! Now clean up the rust that you've uncovered with a abrasive wheel and POR-15 and recover with your choice of sound deadening and carpet!