Westfalia made OEM hitches for the E53. I believe they made one hitch for the US and one for Europe. I do see the European hitch on Euro Ebay sites occasionally. This is the hitch to get. All the other hitches cut corners here and there and result in lower towing capacities. Some may actual damage the vehicle. I recommend OEM only for a hitch.

There is a OEM hitch DIY on-line, have a search.

As noted above, you should not attempt to wire in a generic tow harness into the tail lights. This will likely end in big problems. Find the OEM tow module and tow harness (loom).