The input capability is already built-into the LM-1 (http://www.innovatemotorsports.com/...tm#_Toc46234170), but I guess you might want the special cable to interface to the Mini-DIN 7 plug. I'm sure you could come up with your own interface by visiting a Radio Shack. You'd just need to grab an rpm signal voltage from somewhere in the car. It simply records the voltage and then you would need to convert that to an RPM in MS Excel.
BTW. An 8 pin cable will work also in the 7 pin Plug of the LM-1. Get a Macintosh serial cable with all 8 pins connected and cut off one end. If you have an analog RPM signal already in your car, you do not have to convert it using Excel. The LM-1 software can do that for you. For the input used, just specify the unit (rpm) and which rpm is equivalent to 0V and which is equivalent to 5V. The produced Excel-log and graph will show the log already converted into the specified unit. We are working also on a future software release (no release date yet) that will allow you to link to sensor output curve files (excel files), so that the log download will do the linearization for you for non-linear sensor signals.
The upcoming aux-in cable with RPM allows to hook it up with an inductive pickup (included) or you can directly hook it up to a crank synchronous or cam synchronous pulsed signal (1 pulse/rotation). That signal can be anywhere from 300mV to ignition primary (not tested yet, but will be before release).
The aux-in cable has a built-in switch with which you define whether the rpm pulses are 1 pulse/crank rot. or 1 pulse/2 crank rot.