you can convert the signal with two relays:

relay 1:

85 coil: ground
86 coil: positive stock signal lead 1
30 common: green wire on actuator
87a N/C: ground
87 N/O: +12V constant

relay 2:

85 coil: ground
86 coil: positive stock signal lead 2
30 common: blue wire on actuator
87a N/C: ground
87 N/O: +12V constant

your actuators now rest at ground. when a positive signal is sent from the stock lead 1 the green wire will temporarily be given +12v thus activating the actuator. when the signal ceases it returns to ground. the same happens when a positive signal is sent from stock lead 2 - the blue actuator wire is temporarily given +12v. this activates the actuator in the opposite direction.

you can convert almost any type of signal with a few simple relays.

-marc