Digital waveform input help needed for electronic steering unit.

[irrational]

 Hello this is my first time on this  board and im looking for a little help, or atleast guidance. Ive been going back and forth with this guy in portugal who doesnt speak any english for a control box for my steering unit (EPAS). Here is the link to this mysterious control box http://www.ebay.com/itm/Saturn-Vue-Ion-Chevrolet-Equinox-Electric-power-steering-controller-box-EPAS-/182008071875?hash=item2a6086c6c3:g:5usAAMXQHDlRcSCy&vxp=mtr

 What im looking for is help attempting to re-create my own control box since i bought two of his controllers and they both either started smoking or stopped working for whatever reason. And his "help" on this situation is downright terrible.

 So apparently this controller replicates the signal from the cars ecm for two channels. One for when the engine is running and another for what speed the vehicle is traveling. Ive read that one channel requires a straight 500hz and the other channel requires a starting 500hz and then adjusted up or down to 10khz for the optional steering input assistance level AKA how easy it is to turn the steering wheel.

 Anybody have any suggestions on how to makea circuit that can adjust the output linearly up and down from a simple potentiometer? Or perhaps even a DDS like this one? http://www.ebay.com/itm/DDS-Function-Signal-Generator-Module-DIY-Kit-Frequency-Range-1-10000MHz-US-/171812317597?hash=item2800cfe99d:g:FwIAAOSw6EhUN7m-

 I appreciate the help, and the help you give me will help many others from the automotive community when i figure this out.