well icen i was thinking of something like that but i feel a relay would be a little better since i read a little about them since i had some free time im not really sure how diodes work but we'll see.
as you can see from this pic i got from the12volt.com i would connect the positive cable to the led lights to the common terminal (30) and have the fading power input from the domes connected to terminal 87a (which is active when 85 isn't energized) and then have the switched power connected to terminal 87 (which is on when they are energized). 85 would be connected to the ignition switch or accessory and 86 being the ground connection
the only problem might be that in all the diagrams i've seen i only see 30 as being the only power supply supplying power to 87 and 87a i haven't seen and diagrams using them as the power supplies and 30 being where the energy is going.
icen if you could detail how diodes work and maybe even a little diagram of how to connect it would be great. i understand how to connect it in series but i just need a little clarifcation