OBDII PORT, CIG LIGHTER NOT GOOD AFTER HOOKING 12V BATTERY TO OBDii PORT
 

2003 C240 87k miles

While swapping a battery I read you can use obdii port to hook a power source so as to keep computers supplied. Hooked up and obdii cable with alligator clips at the other end. Soon as I plug it in theres a beep beep sound. Well I had a 12v car battery and used that. While removing the old battery the +ve did touch the body and there was a slight spark. Completed the install but now my obdii port and cigarette lighter are both not working. I looked at all the fuses under the hood (driver side) and on the driver side dash end but they are all ok. What am I missing? Also is it ok to hook a 12v car battery into obdii port or should I hve used a 9v battery instead?

Whoever told you to hook up a battery to the obd2 port doesn't know what they're talking about. If you want to maintain power during a battery change the car has a charging port right next to the battery under the bright red cover with a + on it.
http://wardsweb.org/cars/C240/distribution.jpg

Yep, an inimical act on my part.

Where do I start to fix the non working OBDii port?

Check for blown fuse in the rear panel as well. Otherwise you're going to have to start tearing out the dashboard and look for melted wires.

Using the OBD 2 connector for keep alive power will "work" and cords are sold for this purpose but I'd use a non switched power port / lighter socket / power distribution terminal first.

For an OBD 2 connector,pin 4 is chassis ground , pin 16 is positive, fused, unswitched battery power for test equipment use.

The slight spark won't cause the dash wiring to instantly melt. Look for a fuse that cleared and replace it.

Three 5 amp fuses were blown - two infront and 1 in rear (actually I think this was a red 7.5?) All is well now, and never again.

I'm surprised 3 fuses cleared. Generally something like is only going to have a single small fuse then maybe a much larger ( like 50 A ) ahead of this bus bar feeding a bunch of other smaller fuses.