7-pin version is the cheapest version with these basic trailer features:
Taillights
Brake lighting
Blinking light
Fog lights
Trailers blinkers / monitoring
Connection of a permanent plus or charged line to additional functions in the trailer and the function of the taillights in a bicycle rack (legal requirements) is not possible.
The 13-pin kit provides the option for further convenient functions such as lights in the trailer, refrigerator operation or reverse light with a cycle carrier:
Rear lights
Brake lights
Turn signal lights
Rear fog light
Trailer furn signal light control
AND ADDITIONALLY:
Reverse light
Permanent positive line
Chaging line
This equips you for all situations from the start.