The reason a simple circuit won't work is that the '03 - '04 switch is only a momentary contact type. You would have to hold it in to maintain power to the relay and defrost grid. Not very convenient. What I proposed is a latching relay circuit that does the equivalent of holding the button in once you press it. The downside of this approach is that the only way to turn off the defroster, once on, is to turn off the ignition. These grids draw ~20amps and are not designed to be operated for more than 15 minutes continuously. In addition, the extra drain on your alternator will shorten its life. Unless most of your trips are 10 minutes or less, I now agree that Svttech76's suggestion to tap into the old switch is probably a much better idea (this is not the first time he has a better idea than I do). Assuming the '99 switch is also momentary contact, its just a matter of bringing out a single wire, which would then go to the '04 switch. The '04 switch and indicator would then operate just like the '99, and you would not need any relays or diodes. The old switch assembly could be taped to a harness behind the new one. If you would prefer not to take the switch apart, PM me and I can get it done for you (I work in an electronics lab where we design automotive electrical parts). Your only cost would be 1 way postage to the East coast.
old thread, but this comes up as a top hit on a search.
The 94-00 defrost switch has an jnternal relay as part of the switch. The 01-04 switch activates an external relay. What I did was just extend wires from the 01+ switch out to the relay on the 94-00 switch and removed the push button components.
If you have done, or are considering doing a double DIN swap from an 01+ mustang into your 94-00 mustang and want to complete the transformation, you're probably like me and dig the smooth look of the cluster bezel without the eyesore of the rectangular defrost button. Not to mention, having blan...
www.sn95source.com
I removed the push button and yellow LED from the original switch and soldered wires onto the correct leads to extend down to the newer button.
Here’s what’s inside the 94-00 switch. The yellow LED, push button and blue bulb get removed and the housing trimmed.
And in case the above link doesn’t work, here is where you are extending wires from