Here is something to think about.
On other projects we have created what is called an SSR(Solid State Relay). we used a combination of an opto-isolator and a MOSFET to switch high current, high voltage loads with 5V signals from the computer.
So, you could use something like these parts together to create your motor ssr.
These parts I found at Jameco.com, but you can get them anywhere.
MOC3062 PhotoTriac Driver Optocoupler: MOC3062M: Fairchild Semiconductor : Optocoupler Triac AC Output 1 Channel 600VDRM 6-Pin PDIP White Bulk : Opto & Illumination
IRF540 - 100V, 27A MOSFET: IRF540: Major Brands : Transistor IRF540MOSFET N Channel 100 Volt : ICs & Semiconductors
The optocoupler is used to isolate the MOSFET from the low power circuits(Arduino). The MOSFET actually does the switching for the motor. The triac side of the optocoupler is on the same power line as the motors and the MOSFETs. The resistors are current limiting resistors to protect the parts from getting too much current and burning out.
Arduino —> resistor —> Optocoupler pin1 —> optocoupler pin2 —> Arduino GND
+12V -----> optocoupler pin6 and motor+
optocoupler pin4 —> resistor ----> MOSFET gate
-12V -----> MOSFET source ----->MOSFET drain -----> motor-
I hope this doesn’t confuse you too much. I could do a drawing later if you need one.
Hope this helps. :-/