Thinking about your project, I have done a similar one. Most Honda factory stereos have backlighting illumination when the headlights are turned on. This wire is the Red/Black lead on the Accords (maybe the same for you). Too make a long story short, when I installed my aftermarket stereo, this lead was not used. I connected this lead to an led (with a series resistor), but a 12V led will work because of the internal resistor and grounded it.
The basic purpose was to add some blue backlighing to the aftermarket stereo installation pocket.
But, I had to be 100% sure that this will cause any electrical problems, so I verified it by examing the manual for 90-93 Accords. No problems, but I added an in line fuse in case if some short circuit.
Your project should include an inline fuse. The fuse and fuseholder can be bought at radio shack for about $3. Since you are using LEDs which are low current, 0.5A fuse will work (AGC type or automotive glass type) with the 1 X 1 1/4 fuse holder. You can also use 1A ATO fuse and the ATO inline fuse holder from Autozone.
Remember, you dont want to be in traffic and your car stalling from some electrical overload, adding some circuit protection to your wiring wont hurt.