Other ways to convert - to + for door locks?

Is there any other way to convert a negative signal to a positive signal without using relays for power door locks?  Crimestoppers sells a device that they call a pulse inverter...that does just that but I want to know how.  It is only $4 and I know for sure it isn't a couple of relays because of the size.  Here is the website so you can see for yourself

http://www.logjamelectronics.com/crimcs6500dl.html

For people not familiar with Crimestoppers...off of the brains there are 3 wires for the door locks, 2 negative out for lock & unlock and they also have a positive wire for reverse polarity door locks...but I am not sure if it is a constant power out or just when it is needed.

You could use two PNP transistors and a few resistors to accomplish this.  It's pretty basic electronics,   your just using the transistor as a switch.   The negative lock or unlock signal gets fed to the base of the transistor via a resistor (1K - 5K),  The Emiter goes to 12V and the Collector would now be your Output.   The Base can also have a 10K pullup resitor to 12V.   Probably a buck or two in parts.