single positive pulse to constant on/off?

Isn't this a mechanical latching relay that only requires one pulse to alternate?

I think so - it's the "standard" VW/Porche hi/lo relay.

Why not a cheap simple OMRON latching relay, available from R/S here or Farnell and probably Mouser in the US, you have 1 and 2 pole changeovers, you can feed either a constant 12volts or ground and switch the other side of the coil via - lock triggers (or of course vice versa), will give you a constant pos or neg going on way when car is locked an going a different way when unlocked with no battery drain.

DEI relay. I've used in with an aux out from my car alarm to trigger strobes. Pulse it once turned the strobes on pulsed it again and it turned them off. Google Pulsed latching relay http://cgi./DEI-611T-12-VOLT-ACTIVE-LATCHING-SWITCH-RELAY-TIMER-/140333447156#ht_2711wt_905