This is simply advice from an American....to other Americans....when it comes to staying in Germany and trying to take the US car into the German registration business.
First, do everything possible to avoid it. By the end....you would prefer a root canal job, over maneuvering your US car into a German registration deal. Buy the Euro spec's car and be happy with no hassle from the TUV guys.
Second, if you decide to go through with this....find yourself a top-notch local mechanic who has done this before. He knows the checklist and knows generally how to ask for waivers. Utilize any and all waivers possible when it comes to changing things on the car....from the headlights, to the safety equipment. The TUV folks do allow waivers, but you need to ask and be diligent on asking for just about everything.
Third, accept the fact that this might run into the 2k to 3k Euro range, and really not be worth the cost in the end. If you think your car is only worth $7k now....don't waste your money. If your US car is worth $18k....and mostly new.....it probably is worth the effort.
Fourth, the sad truth is that each region's TUV office is different and takes to the rules and standards differently. You could pass one vehicle through a Bavarian area for less than 1k Euro in cost. The K-town area? It might be 2.5k Euro in total cost for the same vehicle.
Fifth, once you get it through the system and tagged....don't expect a fair resale value. If you changed the vehicle drastically....it's just not worth the same amount of money. So you'd best think of this as a car that you'd like to keep five more years as a minimum.
Sixth, pick-ups have a odd value in re-sale after you put the German tag on. You might actually find some local German who a year or two later....wants a Ford F-150 pick-up for the actual real value. So take that for simple advice.
Seventh and final....TUV is final. If they say a drastic change is absolutely required and a waiver won't be given.....that's the end of the discussion. The rules run into the hundreds of pages and various rules could be read in different ways. Don't take it personal if they really slammed your vehicle and put an entire day of mechanical work onto your tasks.
US vehicles sold as new? Yes, Dodge and a couple of other car companies have made an effort to sell a couple of US-made cars on the German market.....already with full-given TUV. If you were into this US-car thing and had a year or two before messing with German registration.....maybe this is the better method of getting your vehicle. My own advice is to buy a German or French-made vehicle and just be happy with no extra paperwork.