Wire courtesy lights to dome light

[Jay11]

1998 Ranger 2.5 Regular Cab
I want to wire door courtesy lights to the dome light, that will light up exactly the same. I see a green, yellow trace (GY) and a black, blue trace (BB) wires at the dome light. I was wanting to tap into the wire loom on driver side, behind the seat, just before where it goes up the wall to the dome light (pic 2). So I tapped a test light into the same wires. Even though the dome light is working just as normal, my test light isn't working. Can somebody assist, what am I missing?

 

[RobbieD]

Put your test light clip to ground. You should see constant +12 volts on the Grn/Yel when probed.

Ford switches positive to the courtesy light. Test light clip grounded, you should see + 12 volts on probing the Blk/LBlu wire, when the doors are open.

 

[Jay11]

I had 5 minutes when I got home this PM, placed the tester on the B/B wire and boom, it worked flawlessly.

Now then just for better understanding as my 2 brain cells could not follow what you wrote here.

Put your test light clip to ground. You should see constant +12 volts on the Grn/Yel when probed.

Grounded, tried the tester on the wire's spliced part behind the driver seat, tester light never came on. Even tapped the yellow screwed part on the dome light + a ground, no joy. If I understand correctly, this should make the tester light up constantly disregarding if door is open or closed. Correct?

Ford switches positive to the courtesy light. Test light clip grounded, you should see + 12 volts on probing the Blk/LBlu wire, when the doors are open.

Now I half get you-the Black/LBlue is the switched positive, thus, the tester mimicking the dome light with door action. And that's the holy grail of this pursuit.
After this install I gotta do the same on the F150. Think I was told to look for a purple wire along the driver side door sill.