There are 2 expansion valves for limousines (at least for properly built ones). One in the front engine compartment for the front area, and 1 in the rear (usually under the long bench seats). Replace the 1 for the rear.

P.S. Since you will have to purge system of all freon to replace the valve, now would be a good time to service anything else with the A/c since the lines will be open. Just a tidbit, kill as many birds with that one stone. Good Luck!!