How hard is it to tie new led lights into the existing interior lights?

 

Not hard at all. All you need to do is find LEDs rated for 12v (a lot of strips will be able to run at this voltage but you want to avoid LEDs rated for 5v or 3.3v as they will be fried by the cars battery). Then all you need to do is connect them to the same connections as the factory lighting, just watch the polarity as LEDs don't illuminate when they are connected backwards. You can choose whether or not you want to leave the factory lights in along with the LEDs or remove them.