Special Relay setup?

Here's what I want to accomplish.  I want to set up something that will emit a single pulse (ground) from two sequential pulses (positive)....  If the second pulse is not completed within a few seconds then the circuit will not complete. 

I've been searching for some kind of time delay relay, but have not had any luck.  The ultimate goal is to make a channel replcator to activate a remote start off of a factor keyless entry.  Another thought is to have some kind of time delay relay on the unlock circut... and a regular relay on the lock circut.  Press unlock, and the time delay keeps the half of the circut closed for a second, and then hit lock and a regular relay completes the full circuit.

could this work? where could I get a relay that would stay active for a few seconds??

thanks

Jeff

Would this work:

Lets say that this is activated by the unlock circut from the factory keyless entry. This relay is ganged with another relay that would be in series with the first(delayed) relay. Then while the first relay it is active, if the lock button is pressed it will close the second relay which will ground the pulsed input for the remote start and thus start the car.

-Jeff