The wires on my 08 Mariner are too small to splice and I myself would not even think they could handle a load from any trailer (just because of the size of the wire) and I'm not sure of the wire sizes of the other vehicels on this site.
There is no T- connector to plug into, from what I remember my harness contains the taillights, backup lights , backup sensors, and grounds, and now the trailer wires, that plugs into 1 connector that has atleast 12 wires or more.
Thats why you can use a converter , or add to the factory harness, The relays are required to run the factory trailer harness.
Here is the discriptions from my power distribution box. (under the hood)
Fuse #8 20 Amp Trailer tow parking lamps
Fuse #14 15 Amp Trailer tow turn lamps
Relay #31C Trailer tow left turn relay
Relay #31D Trailer tow right turn relay
Relay #31E Trailer tow park relay
The trailers I have is a 5'x8' Alumna (brand) and a 40"x8' homemade that I'm rebuilding to haul a motorcycle, both have led lights to keep the amp load low.
Agian I hope this helps, and anyone else that can chime in here and help me out please do.