SIMPLE ICC SWITHCNOTE - Newer D's have an ICC switch.
Amongst trailer truckers it's considered common courtesy to signal a passing trucker (by flashing the headlights) that it's OK to pull back in line.   Likewise, it's considered common courtesy, after you have passed another trucker and he has given you the headlamp flash signifying OK to pull in, to turn the rear running lights off and on as a "Thank You".   For many years these signals were generated by simply turning the headlamps off and on using the headlight switch - something the switch wasn't designed to do and this lead to a lot of headlamp switch failures.   This problem lead to the "invention" of an "ICC Switch" which is a spring loaded toggle switch placed in the lighting circuits that allows the driver to flash either the headlamps or tail running lights.   Unfortunately, it's pretty rare for RVs to use these signals, and if we did our relations with truckers would improve tremendously.   It is also unfortunate that the D does not come with an ICC switch as this is standard on most coaches.   The following article describes how to install a simple ICC switch on your D.   This simple modification only works during the daytime, but is still a worthwhile addition.
I installed an "ICC Switch" on my '99 37V as follows:
There are two large white wires coming out of the headlight switch - one powers the headlamp circuit and one powers the running lights.
Parts from Radio Shack: 2 heavy-duty 12volt relays - they were Single pole, single throw and I think rated at 30 or 60 amps.   The relay configuration is a small black cube with two tabs for the relay coil and two more for load in and load out.   The switch was a spring-loaded Center off double pole double throw automotive type toggle switch.   Look at the 12v relays and pick out the
heaviest duty one they have.
I mounted the switch where the level used to be on the console to the left of the driver's seat and oriented it such that pushing it towards the front of the rig flashes the headlamps and pulling it towards the rear activates the running lamps.
Wiring up was fairly simple:
I just used the relays to "Jump" around the headlight switch.   To do this you'll have o take of the cover behind the instrument panel.   The headlight switch and wires I described are easily available.  I used the heavy-duty "wire tap" connectors rather than cut the original wiring.   So, you pick up the 12 volts from the 12v feed to the headlight switch.   This is 12 volts at all times.   Run the 12volts to relay load in and to the toggle switch.   Run a wire from the toggle switch to relay coil lug one.   Ground relay coil lug 2.   Run the 12 volt "Load out" side of the relay to one of the white wires coming off the headlight switch.   Repeat for the other large white wire coming out of the headlamp switch.
One comment: This setup functions as an "ICC Switch" in daytime only.   A true ICC switch has more relays to sense whether you have the headlights on (running at night) or not.   This added feature complicates the whole deal.   I don't drive the D much at night, and when I do I simply use the headlight switch to do the signaling.
This setup has been on my '99 37V for 2 years and works trouble free.
Submitted by Fred White