12v source only when engine is on

The pulses are not the problem, the 90 Ohms resistance of the relay's coil will probably be a bit much of a load for the circuits to drive.  You can connect it briefly to the tach wire and see if it makes the car idle rough.  If it does not affect the running of the car, then yes the diagram I posted on the other thread may work for you.  If it does make the engine idle rough, we may be able to rectify that problem with a transistor. 

just use a relay in conjunction with the oil pressure switch on the engine. if you only have a gauge sender and not a switch, you can pick up a simple 1/8" pipe N/O oil pressure switch at any decent parts store and any fitting you need to plumb it in. engine is running=oil pressure=switch will turn on relay. simple.

You can pick up a frequency switch pretty cheap if you do a search on the net.  You can then hook the frequency switch up to a fuel injector - have it activate a relay if the fuel injector is cycling.  That is probably the best way to actually tell if the car is running (that or a cam or crank angle signal feeding the frequency switch).