I dont know if you keep the unlock but. down (hold) the pulse out will be as long as you hold? or just a pulse..? if is as long as you hold, is better to use the 555 based on the hold of the 12 volts, so you have less posible truck opening by mistake...now if the 12 v pulse dont hold, I will use the 555 and a counter at the output so after 3 unlock signal in a time window of 3 seconds the truck open...4017 counter can make it and no need of 5volts as cmos, dont forget to drive the solenoid thru a relay , and put a reverse diode in the relay coil terminals....with the 555 enable the 4017 for 3 seconds, if it count 3 pulses , it open by the respective output , filter the clock input (pin 14) with a cap and resitor to clean the pulse in, not to big cap....the output 2 (pin 4) will be high after 3 clock pulses, this output should have a pull down resistor of 10 k....you got it?? this way you have a fail proof system....no false truck openings, a bit more thing, but a way more confident....