Looks like I found the issue regarding the dimming, or lack thereof..from avic411.com
Your scenario about where to get an illumination tap is valid on most vehicles. But there are a few where tapping into a park light, etc just won't work either. We just went through this on a Lincoln LS. All the lights are multiplexed and have power to them at all times. It uses the ECM in conjunction with the GEM, REM, etc to control the lights. Once the computer sees a signal from the switch, it orders a module to ground the light being commanded. Therefore, you can't just hook into one of the lights. It always has power there and is just waiting for a ground. What we ended up doing was using a feed off the ash tray light. That was conventionally wired. We also had the same problem with a reverse signal. We never got that one resolved. Mulitplexing (aka twisted pair) is such a great idea, but it sure screws up conventional wisdom when it comes to these sorts of things.
Your scenario about where to get an illumination tap is valid on most vehicles. But there are a few where tapping into a park light, etc just won't work either. We just went through this on a Lincoln LS. All the lights are multiplexed and have power to them at all times. It uses the ECM in conjunction with the GEM, REM, etc to control the lights. Once the computer sees a signal from the switch, it orders a module to ground the light being commanded. Therefore, you can't just hook into one of the lights. It always has power there and is just waiting for a ground. What we ended up doing was using a feed off the ash tray light. That was conventionally wired. We also had the same problem with a reverse signal. We never got that one resolved. Mulitplexing (aka twisted pair) is such a great idea, but it sure screws up conventional wisdom when it comes to these sorts of things.