[Michael23451]

Splicing in the trailer wiring harness was my biggest concern. However, it was the easiest part as it turned out.

The CURT 59496 Powered 3-to-2-Wire Splice-in Trailer Tail Light Converter Kit, 4-Pin Wiring Harness has the following cables:

Vehicle side

GREEN - connect to GLA right turn signal - black and green striped wire

YELLOW - connect to GLA left turn signal - black and yellow striped wire
BROWN - connect to GLA right tail light - black and grey striped wire
RED - connect to GLA right brake light - yellow and white striped wire

WHITE - connect GLA Ground - any metal screw
BLACK - connect to GLA 12V - Black and white striped wire of rear power outlet

I soldered the connections. You may choose the included hardware or wire nuts.
Route the green wire to the right turn signal BEFORE making the connection!
I also included the provided fuse (red) between the power adapter and the black CURT power cable.
Because I will rarely be pulling a trailer, I keep the connector in the trunk, and only take it out when needed. If you do the wiring at the same time as the hitch install, you can attempt to route it through the bumper. You may need additional parts for that, and drilling may be required.

Here are pictures of the wiring:

CURT 59496 Powered 3-to-2-Wire Splice-in Trailer Tail Light Converter Kit, 4-Pin Wiring Harness

Routing the green wire (step 1)

Routing the green wire (step 2)

Connecting the green wire to the right light (step 3)

Hiding the green wire (step 4)

Yellow, red and brown wire connection to the left light

Ground wire

12V wire with fuse. Power supply can be seen in the upper right.

All tucked in. See the power supply again in the upper right.

This works for me. You may like to route the cable through or below the bumper for a permanent setting.

 

[Michael23451]

I hope someone will find this useful.

Also see this:
Hitch Installed on 2017 GLA 250...
Mercedes GLA Hitch Wiring Thread
2015 GLA45AMG Trailer Hitch Install