A pulse is basically a digital signal. ON for a given amount of time.
Typical Relay functions could be: lock, unlock, starter disable, light flash, trunk unlock
Two pins should basically be accessible to the user. So these relays could be SPST, so there will be 2 terminals for the contact and 2 for the coil.
These relays: **broken link removed** or "Bosch automotive relays" are very common for alarm installs.
You may have to use one or two especially if you want to use a low current easily hideable switch for lock or unlock.
Take a piece of hose and listen to the relay (act like a stethescope) when the lock and unlock pulses are sent to identify.
Depending on the vehicle, it may use a contact to ground or a contact to positive. Way back when (60's) Ford switched the positive for the dome lamp and Chevrolet and Chrysler switched the negative. Don't assume.
In this case, your going to want to parallel a set of contacts across one of the existing relays. The current draw of a lock system may be unknown. Very old cars used large solenoids. Now, the way to do it is to use a motor that drives a rack and pinion.
In order to have a "small switch" activate a "large load", you probably should use an Automotive relay to get that 30A rated contact.
Relays can have terminals labeled NO for Normally Open, NC for Normally Closed and C for common. The NO position is where the Common contact and the NO contact are not connected when power is off.
You can also place a voltmeter ACROSS the relay contact. When the relay isn't energised, you will see 12 V. It will blip to zero when it pulses.
This kind of use of a voltmeter might confuse you, but the circuit would consist of the battery + terminal, load, the relay and ground. Since you meater draws very little currrent to measure voltage, the actuator looks like a wire with the relay being the open in the circuit. Measuring across the relay will show 12v. When the relay is activated, the leads of the voltmeter see nearly 0 resistance with a large current and thus reads zero to a few mV of voltage.
Voltage measurement from ground to the relay contacts and also from + to the relay contacts can help you figure out what side is switched.