Do you want the lights to work only with key in ignition and on accessory or run?
Sometimes tapping existing wiring and asking that originally undersized circuit, to power even more load, even within the rating of the fuse, can be less than Ideal and become troublesome.
While one will often say well LEDs draw less juice, some of My LED T10 wedge base bulbs can go as high as 0.7 amps each, which is insanely bright and more amp draw than a 168 or 194 incandescent, but I use voltage buckers as a dimmer, so I have that insanity on tap, but dial it way down 99% of the time.
My voltage bucker is rated for 5 amps, so I need to keep all the lighting powered through the bucker below this threshold which has not been an issue.
If you tap an existing circuit to power your new lighting harness you either tap it before the fuse and fuse your new circuit separately, or after the OEM fuse and hope the total load on that circuit stays below that fuse's rating. You cant just use a higher rated fuse, safely.
My preference is that my lighting be independent of the Ignition/ OEM wiring. My door activated dome lights are still functional, but on my Dodge, there is a time delay relay on this circuit which will cause an almost 0.3 amp parasitic draw even if the dome lights are switched off at the dome light, but the doors are open. To eliminate this parasitic draw I have to rotate the headlamps dimmer switch all the way off.
0.33 amps for 3 hours is 1 amp hour consumed from the battery. a group 34 starting battery has ~55 amp hours total capacity, when new and fully charged.
its not a huge parasitic draw, but it will add up to the detriment of the battery, which ideally always wants to be fully charged. *0% charged to 1005 charged takes no less than 3.5 hours when held at mid 14 volts that entire time, something few voltage regulators allow.
Tapping a fuse for another circuit is not hard. The following products make it easier:
When touching original wiring is not desirable, and running new wire through the firewall not a huge ordeal, I recommend doing so and using a new inline fuse at the battery with 10AWG running to the interior.https://www.amazon.com/10-Gauge-Fus...d=1622271840&s=automotive&sr=1-5
The quality of your wire terminations will be directly related to the reliability.
If running a new fused circuit through firewall, then products similar to below can also be useful:https://www.amazon.com/Sunjoyco-Cha...=1622272074&s=automotive&sr=1-27
Dimming LEDs is another topic, and PWm LED dimmers are easily available, but in my experience the LEDS can actually whine audibly as reduced brightness and can screw up radio and TV reception.
I'v replaced all my PWM LED dimmers with xl4005 or xl4015 based voltage buckers, as they can dim much further and they do not have whine when dimmed and do not flicker, as many PWm led dimmers and PWM motor speed controllers do. The dimmers are nice as they have the adjustment knob built in, but one should shop for PWm dimmers or controllers with 21khz or higher to prevent audible whine to human ears, though Fido might still hear it. The PWM dimmers also work with incandescent lights if one is not inclined towards LEDs
The voltage buckers I use I Modify with a remotely located fingertwist potentiometer of the correct value, as otherwise one would need to bust out a jewelers screwdriver to adjust.
I use these Voltage buckers to speed control insanely powerful computer fans as well.