Back in the old days, you could have easily ‘just tapped into’ another circuit, (such as the running lights) w/o any issue aside from potential wire size/current limitations and fuse ratings (which were almost always over rated for safety).
I suspect the issue that you may run into with ‘today’s automotive electrical systems’ is that they have become increasingly more complicated in their attempt to ‘make things easier for the consumer’.
This means lighting circuits are now computer controlled and monitored for load/current changes to alert us to abnormal operations, burned out bulbs, etc.
By tapping your LED DRL components into the existing running light circuit, you are adding a parallel component to the existing load, which decreases the overall load (as opposed to a series circuit which would increase the overall load).
The decreased load, MAY be different enough that the computer sees this as a blown bulb and it will act accordingly (based upon circuit design parameters).
A good example of this condition is when someone adds an LED turn signal lamp in place of the traditional (original) incandescent lamp… the system measures the reduced load and believes there is a blown bulb; hence the frequent “hyper-flash” that so many have dealt with (which typically requires the addition of external load resistors, and/or different flasher modules to correct).
BTW, many of the newer (more expensive) LED lamps out today have this additional resistance already built into their design, and for basic automotive circuits, this is sufficient to eliminate the hyper flash. Others (think CANBUS), are more complex and may still not function as expected after an LED “upgrade”.
Since the running lights don’t flash, this likely won’t be a problem, but I’ll bet the RL circuit is monitored by the ECU/PCM, and you could get a fault indicator or a code due to the reduced load you created by adding the parallel device.
In short, you are far better off powering your DRL’s from a specific power feed than you are tapping into another circuit.
I added DRL’s to our ‘03, and was able to make them function as intended with the addition of a simple DRL module, which monitors the voltage at the battery, shutting the lights off when the voltage drops below about 13-14 VDC.
This device is basically creating a switched feed when the engine is running and battery voltage exceeds a specified threshold (say 13.8 VDC), making it very similar to an ignition controlled circuit, without the hassle of tying into an existing circuit.
You can buy the DRL module from Amazon, eBay, etc for about $10.
When you shut off the vehicle, the alternator stops charging the battery and the residual voltage drops below the threshold (predefined by the DRL module) within about 20-30 seconds. This leaves your DRL’s illuminated until the module detects the drop, then it shuts them off.
I know this was a bit long, but it is the right way to install DRL’s on a vehicle that didn’t come with them from the factory.
