My new x5 tow bar has an unusable electrical adapter. The adapter has a rectangular black box that should be ... Perhaps Duct taped to the trailer ... To prevent it bouncing along the road as I drive.

Congrats, you have found Ford's super secret fuse that drove everyone mad for years!

The X5's and some 5 series models have a compressor in the boot which pump up the rear shock absorbers to level the rear of the vehicle when you load the boot or connect a heavy load on the tow bar, before going down the road of getting an expert to look at it have a look under the vehicle and you will see to black boxes one each near the shock absorbers and check the arm that comes off these two boxes they should both be at the same angle in relation to the black boxes if one is different then its causing the system to be out of alignment.

I assume its a new W212 E-Class ? Do you need a 7 pin or 13 pin socket. Either way adapters are available from MB and these have to be programmed at the dealer so the CAN system identify's what the new accessory is.
I would check with a different dealer if you are not getting anywhere, or a reputable tow bar fitter.

if you go to the parts store they make them ready to plug in you will have to just unplug the tail lights and plug the wire harness inplace of it then itll plug back into the original spot.

Not sure which fuse you checked.
The Engine Compartment Fuse Panel ( Battery Junction Box / BJB ) is where the fuse is for the trailer tow adapter parking lamps.
Fuse # 5 is the fuse for the parking lamps, and the reversing lamps.

Next is the trailer tow adapter park lamp relay. This is what BJB Fuse #5 feeds, and the coil is operated by the truck's parking lamps. This is also in the BJB, Relay 201, it is the 1/2 size relay.

Not sure what you took a resistance reading from on the trailer tow adapter.

If you want, you can pull the trailer tow park lamp relay, and jumper the normally open contacts, which should provide power at the trailer tow adapters, regardless of the truck's lights.


This is the wiring diagram for the trailer tow park lamps

There are a few fuses for the trailer tow adapter.

1st, is the odd location on the 97-98, engine mini fuse panel has a 20 A fuse for the stop / turn lamps. Have seen it covered in tape from the factory on some 97 MY trucks.
This is on page 142 in the owners manual.
If you see one, you can download a PDF version from Ford at :

https://www.fleet.ford.com/maintenance/owners_manuals/default.asp

The others :
1. Power Distribution Panel / engine compartment fuse panel
Fuse #1 - Trailer tow back-up and tail lamps
Fuse #16 - Trailer tow battery charge and stop/turn lamps
Fuse #28 - Trailer tow electronic brake

2. Instrument Panel Fuses ( cab fuse panel by hood release )
#23 - trailer tow battery charge relay

Perhaps this link will be of help to you. http://www.watling-towbars.co.uk/towbars.html

Battery junction box (BJB) fuse(s):

• 5 (30A) (trailer electric brake controller)
• 34 (25A) (trailer tow turn relays)
• 42 (30A) (trailer tow parking lamps)
• 44 (30A) (trailer tow battery charge relay switch side)
• 63 (15A) (trailer tow reversing lamps

ask your dealer to recode the trailer unit with the trailer connect to the car and in this way the car will see that the trailer is on and will accept it bmw is a funny car.