Converting 4-pin tow connector to 13-pin

Hi, I’m currently in Germany and I need to convert the 4-pin tow connector on my 2002 Subaru Forester to a 13-pin for a bike rack/trailer I want to purchase. Hope someone help me with this?

The standard 4 pin is:
White; Ground
Brown: Running/Tag lights
Green: Right Brake/Turn Signal
Yellow: Left Brake/Turn Signal

Which one is the trailer connection missing? I almost suspect that the Ground is missing on the trailer connection and is using the hitch for ground. If this is the case just splice a 4 pin onto the trailer and bolt the White to the trailer frame. You can use any 12 volt battery and a couple of jumper wires to test which wires on the trailer operate which lights. Attach the neg battery terminal to the trailer frame and use the positive to see which lights come on. On the vehicle side use a test light between the white (ground) and the other pins and see which is hot with the brake, right turn, left turn, and running lights.

I have a 4 pin connector already on the car and need to wire a 4 pin to a 13 pin which is how the bike rack is set up due it being European. Or will I need to go from 4 pin to 7 pin and then use a 13 pin to 7 pin adapter?