It is very easy to get the NAV unit out. The instructions are included with the metra dash kit which you can
download here. I was dreading this part of the install but it was so easy that I was super-confident when it came time to start cutting all the NAV wires in the dash.
First I measured the speed sensor input voltage from the factory wire and it seemed to fluctuate between 0-10.6 volts DC depending on the speed. I feared that the sloped voltage might be critical but then I figured I only needed to emulate the 1 km/h signal so it could probably be fixed. I tested it with fixed 9 and 12 volts DC input signals and both seemed to work although I only tested them briefly for fear of harming my NAV. The final device has a capacitor on the output line which reduces the effective voltage to 0-1 volts DC and it still works as long as the pulse timing is right, so it looks like the voltage is not as critical after all.
I did a few calculations based on some 555 timer IC info which I found on the web. The IC did come with a small sheet but it was only for astable mode and we wanted monostable. Once I knew that the R and C values were close I just purchased potentiometers so I could trim them to exact values as needed. As luck would have it the max settings were right on the money and worked out without tuning, at least on my vehicle.
The breadboard isn't a bad idea but it is a little big and I believe that a mobile component should be soldered, especially if you bury it in your dash. You don't want to hit a hard pothole one day and lock your NAV functions back up...
I am really torn on which mod to complete next. I have the ACME parts and plans ready for several projects including the blind-spot cameras, remote gas door lock, and the power rear window slider for my DC. I also have a top-secret mod which I may not actually install, but if I do then it will be visible for miles around and will help me find my truck in a crowded parking lot at night. Think high power visible green lasers... errr.... I didn't say that....
