Get direction with custom code

If the showrooms are defined as location-enabled nodes (using location_node or location_cck module) and the user's location is known (using location_user module) you should be able to provide a list of nearby showrooms or a map using the Views module. The location module has a proximity filter in Views. Once you have that information you can use a getdirections function to show the map, see getdirections.api.inc for details.
Or you could use Views to make a list of showrooms in a block and when the user clicks on one show a map with the showroom as 'To' where the user can enter their location, 'From' and this will provide a route.