simply way is to make a waypoint from garages to the pitlane,
1)make a new waypoint from garages to your pit lane, closing off one end that not used (end capped it closed).
2) join your garages to the pit lane.
3) if the garages are small Enclose a invisible wall around the garages area along the route to pit lane, that way the aiw will remain in the area required not drive all over the place.(not necessary but safe option - has some ai seem to look for the quick route out.
4) redo Corridors,
5 save aiw file
if you try to join to race track, instead off pit lane. your need to find the waypoints(2) marks in aiw file and re-edited them to Waypoint(1). if memory servers me right,