Disclaimer: the following are very preliminary designs and they're not suppose to be free of problems, and instead, it should have problem, so don't follow them unless you're 100% clear about what you're doing. The author will not be responsible for any problem directly or indirectly caused by the information here.
To control the motor with the guiding signal inputs, one can use a circuit like below:-
If Vcc is applied on dec+, the motor will turn in one direction, but if Vcc is applied on the dec-, the motor will turn in the other direction.
The above circuit can be integrated with the last one for manual control, but notice that those buttons should not be pressed if autoguiding is used.
Notice that dec+/- should be from a relay, not from the guiding signal itself.
It's very cloudy now, probably no chance to take any solar shot today.
Additional remarks on 30/Dec/2007:-
I did an experiment as a test of concept, it works, but the resistor has to be chosen carefully. If it's too low, the current will be large enough to kill the resistor really easily. If it's too high, the current drop will be too large to stop the motor from moving at all.
Therefore, the "internal resistance" of the motor has to be measured first, and then we shall calculate the maximum resistance which will not stall the motor (in series), but still large enough to reduce the current in order not to burn the resistor.