Turn On LED Interior Strip Lights From 2 Triggers

This should be fairly straight forward. But I want to clarify something.

I want to install LED strip lights under the dash and under the seats. Easy enough. But I want them to light up under 2 different scenarios:
1. Whenever the key is turned ON. i.e. ACC power
2. When I open the doors (as a welcome light before I enter the car or put the key in the ignition)

My plan is to install an SPDT relay as such:

87 & 86: to a fused 12V + Constant
30: To the LED lights
87A: to an ACC fuse, piggy backed using an "Add a fuse"
85: to the NEGATIVE wire on the Dome light. (only "ON" when doors open)

2017 BMW X1 is the car I am working on.

My question is how do I determine where to grab that NEGATIVE along the wiring going to the dome light? I understand that this 85 terminal needs to be "triggered" via a ground with the above configuration I am proposing. But isn't the dome light Negative already going to be grounded? Thereby always energizing the coil in the relay?

Dome lights are no longer directly connected to door switches in modern vehicles. The BCM interfaces to both - sensing the status of the door switch and then driving the dome light. The dome light is probably driven on the +12V side. Does the dome light stay on for a few seconds after the door is closed and fade out over a few seconds?

Yes. What you said is accurate. More specifically, if I open the door the dome light goes on. When I close the door, the light stays on for about 30 seconds, then fades out. Also say I am walking up to the locked vehicle and hit the unlock on the FOB. In that scenario the dome light goes on (among other things).

So is what you are saying is I don't want to use a negative trigger in this case to power up my aux LED lights? How would I properly wire a relay in order to achieve my two goals I stated above?

Perhaps an SPST. (just a 4 pin relay - no 87A terminal) as such:

87: fused +12V constant

30: LED lights:

85: Chassis ground

86: Piggy back TWO triggers off this one terminal:

First one going to my fuse box where I can use an "add a fuse" to an ACC port
Second one I can splice into the +12V wire that powers the dome light

Both of these triggers I would apply a diode to as to not send power back to the other trigger.

Does this sound feasible?

Yes, that is feasible.
Alternately, skip the relay and just use two diodes with appropriate current ratings.
I suspect your LED strip lights would draw less than 2A.

Thanks. Is this what you are proposing?

Yes that is it exactly.