I'm not aware of a DIY on changing the 104 throttle actuator. It is of course on the 124 CD. But there really isn't anything to it - just follow your nose, or try a search here. I'm still trying to talk you out of this. The chances of the rebuilt unit being bad are probably as high as of the present one, dirt and wiring excluded. And pin 14 on the diagnostic connector is very likely to tell you what's wrong with the cruise control.