I only looked at the bulldog wiring diagram for your 2007 and compared it to my 2005. I think it said you could use an AC fuel injector wire and mentions needing the db-all. Since i couldn't get the tach mode to work on my vehicle and also because i didn't really need it I just didn't pursue finding a wire that would work. The basic common point that i see between our trucks is that in order to make the tach mode work it needs to be hooked to an AC wire, but it doesn't appear that any of those wires are AC. Perhaps it will need to be hooked to one of the wires on the alternator itself. Maybe you could find out what wire goes to the actual tachometer in the dash from a wiring diagram.

 

You use the negative trigger side of a coil or injector, for the tach signal.

How are you trying to program the tach?

If yours is like the last decade of DEI remote starters that I have used, then you start the vehicle and within 5 seconds press and hold the valet switch, at which point the LED will turn on solid for a second or two indicating the tach has been learned. One of the easiest on the market.

I don't trust too many modules for providing the tach signal. I find many delay the output of the tach and can cause over crank issues.

I don't recall a difference in ignition systems between '05 and '07 when working on the Dakotas.

--==EDIT==--

I just looked at the install guide for the Viper 5901, and it is the same as I posted above.

You could try changing the jumper for the tach threshold, to see if that makes any difference, though it should not be required to do so.