Use a Posi-Tap on either the parking light (if you want it on/off with the key) or the low beam (if you want it on with the engine and off with the key) for your relay trigger.
If you want to use the parking light as a trigger, you could remove the CanBus helper and just run both trigger wires into the same Posi-Tap.
Here's a more detailed explanation of how it all works:
All the parking light adapter does is make it so you don't have to cut/splice any wires. Helpful, but nothing spectacular.
These days, I'd just use a Posi-Tap and be done with it.
Relays use very little current on their trigger wires, and the bulk of the load is fed through the main contacts (that's the whole point of a relay, after all).
The tail light feed doesn't work well as a relay trigger because it's not a standard 12Vdc signal. BMW uses a single filament bulb for both the rear running lights and the brake light.
The rear running lights are fed a PWM (Pulse Width Modulated) signal. Think of a square wave where the signal is being switched on and off rapidly, and thus the average power output is lower than full-on 12Vdc. This gives you a reduced brightness at the rear lamps, but also makes the relay trigger unhappy.
For brake lights, the PWM signal is switched to full-on, giving you full brightness through the same bulb filament.
The low beam is fed a solid 12Vdc signal. Yes, it's monitored by CanBus for expected current flow, but the relay trigger pulls so little current that it doesn't cause any problems.
As an aside, the PWM signal is also why these bikes don't like having HID upgrades on the high beams. CanBus modulates the high beam feed using a PWM signal, to reduce brightness when the bike is stopped or idling (who needs to see when you're stopped anyway). So the HID upgrade kits are expecting a good 12Vdc signal, and instead "see" the equivalent or 8 or 9 volts, so they fail to ignite properly. They will light up once the bike is rolling at speed (and the high beam is up to full power), but then the CanBus notices the reduced current flow (from 35w HIDs instead of 55w Halogen bulbs) and throws a bulb out error . . .
'13 Dark Graphite Metallic K16GTLD, 24K miles and counting...
'09 Magnesium Beige Metallic K13GT, 60K miles miles and counting...
'02 Mauve Metallic K12LTC, 106K miles and sold
BMWLT#145, IBA# 366, MOA# 111996, SCMA# 24032
All lower 48 states plus Alaska on the K13GT in two weeks . . .
Some people see the gas tank as half empty. Some see it as half full. All I care is that I know where the next tankful is coming from...