Why would the throttle rev up while in neutral on a Mercury Topaz?

If you have the 2.3 L I-4, then you probably have a bad Idle Air Controller (they are notorious for going bad). It will cost you about $100 from the dealership for the part alone. It is located on the back side of the intake manifold near the center of the vehicle. It is black and has an electrical connection on top. There are 2 bolts that require an 8mm wrench or socket to remove. There is a gasket between the idle air controller ond the intake manifold. (Make sure when putting the new one on you put it on the opposite direction as you took it off) . It will basically look upside down.You can change it yourself in about 5 minutes.