Its not just the co-ordinates. You can't go south-west from Blitz and get to Heinersburg. Heinersburg is way off in the North of Berlin, North of the Tempelhof airfield.

http://i809.photobucket.com/albums/zz16/FOZ1983/MAP.jpg

Here's a map i was given, I've added to it a little.

RED shows where i believe we were meant to go? I think thats right.

BLUE shows where we actually went haha.

Uopn reaching North of Berlin instead of South, thats when we gathered we were lost so cheated somewhat and summoned google earth to assist us :P

For the culturally interested:
Potdam has no castle, it has a load of palaces, parks and gardens, though.
Cecilienhof is where the Potsdam conference took place, it was a residence for the last German Kaiser.
The palace in the picture is not Cecilienhof, it is the Belvedere on the Pfingstberg, a smaller royal recreational residence on a hill, very close to Cecilienhof. Belvedere from the French good view, because you get an awesome view over the Havel river, islands and hills as well as the palaces and gardens of Postdam and south-west Berlin.

Neither palace was disabled by bombs, don't think they actually were targeted. Cecilienhof, only a couple of dozen meters away from the Berlin wall, was kept in good condition all the time, with lots of tourists coming for a visit, but the Belvedere was closed to the public and left to rot up to 1989, because it did not only give a good view of the landscape, but also of a Soviet military component harbouring thousands of Soviet soldiers, ready to attack the West / defend the East in case the Cold War got hot. It was reconstructed after 1989 and is open to the public now.

lol That blue line is nothing like as squigly as it should be. In the begining I was looking for the airfield at Tegel (where the road E26 is on the map and just south of that small lake that is called flughafensee). I didn't know that it was built in 1948. Thats why we couldn't find it. :-P

[wikipedia] During World War II, the area served as a military training area, mostly for Flak troops. It was destroyed in Allied air raids.

I'm going to check it out next time i'm over berlin.

update
 

Land at the Templehof Airfield, facing West.
Group up, take off turn south.
Head to the town of Blankenfelde.
Turn NorthWest to the town of Potsdam.
Target is north of Potsdam.

Side Note: Will make the mission longer!

http://www.freeimagehosting.net/uploads/967e4d30f4.jpg

http://www.freeimagehosting.net/uploads/4ef3f9fce5.jpg

Target!

http://www.freeimagehosting.net/uploads/7b2ea903d0.jpg