This method has worked fine for me, I have not had any problems whatsoever. Nevertheless, BACKUP YOUR FILES BEFORE EDITING. If you make even the slightest mistake a whole area or possibly even your entire savegame could become corrupted.
Essentially what you must do is unpack the savegame file that contains the corrupted map and edit this map. The savegame files of can be found here (Windows 7):
C:\Users\<username>\AppData\Local\inXile entertainment\Wasteland\<savegame folder>
Contents of GAME1 unpacked
map00.xml (World Map)
map01.xml (Quartz)
map02.xml (Quartz, Scott's Bar)
map03.xml (Quartz, Stage Coach Inn)
map04.xml (Quartz, Ugly's Hideout)
map05.xml (Quartz, Abandoned Building Interiors)
map06.xml (Quartz, Courthuse)
map07.xml
map08.xml
map09.xml (Highpool)
map10.xml
map11.xml
map12.xml
map13.xml
map14.xml
map15.xml
map16.xml
map17.xml
map18.xml
map19.xml
savegame.xml (current stats and inventory of the characters in your party) [1]
shopitems0.xml
shopitems1.xml
shopitems2.xml
Contents of GAME2 unpacked
map00.xml (Sleeper Base, level 1)
map01.xml (Las Vegas, Abandoned Building Interiors)
map02.xml (Las Vegas)
map03.xml (Sleeper Base, level 2)
map04.xml (Sleeper Base, level 3)
map05.xml
map06.xml
map07.xml
map08.xml
map09.xml
map10.xml
map11.xml
map12.xml
map13.xml
map14.xml
map15.xml
map16.xml
map17.xml
map18.xml
map19.xml
map20.xml
map21.xml
savegame.xml (current stats and inventory of the characters in your party) [1]
shopitems0.xml
Preparation for fixing a corrupted map in your savegame:
Download the latest version of the Wasteland Suite (Version 1.3.1 as of this post) and unpack the archive (for example to C:\wlandsuite-1.3.1).
make sure Java is installed. [2]
Copy the savegame file containing the corrupted map (either "GAME1" or "GAME2", see above) to the main folder where you unpacked the Wasteland Suite.
Copy the corresponding Master File [3] to the main folder where you unpacked the Wasteland Suite. If the corrupted map is in GAME1, then copy MASTER1. If the corrupted map is in GAME2, then copy MASTER2.
You should use a good file editor to edit the extracted files. I recommend Notepad++ or an equally good editor of your choice.
Example on how to fix the inaccessible merchant in Quartz (do the above step first):
Open a command prompt in the main directory of the wasteland suite and type in the following command (without quotes):
"unpackgame GAME1_ GAME1"
This will unpack the "GAME1" savefile to the subdirectoy "GAME1_".

Now unpack the MASTER1 file by typing the following command into the command prompt (without quotes):
"unpackgame MASTER1_ MASTER1"
This will unpack the "MASTER1" savefile to the subdirectoy "MASTER1_".

Enter the GAME1_ subdirectory and open the "map01.xml" file with your editor. Now enter the MASTER1_ subdirectory and open the "map01.xml" file in this folder as well.
Between the <actionClassMap> and <actionMap> tags you will see the programmed actions taht correspond to each tile of the map of Quartz. If you compare the GAME1_/map01.xml and MASTER1_/map01.xml on the corresponding tiles where the merchant is, then it will look like this:
GAME1_/map01.xml (savegame map) with inaccessible merchant:

MASTER1_/map01.xml (untouched map) with accessible merchant:

I marked the position in the screenshots that correspond to the tile where the merchant is to be found, you will notice the discrepancy which is caused by a bug in the game engine. In the GAME1_/map01.xml file (of your savegame) there are just dots instead of variables unlike in the untouched master file. The dots tell the game to do nothing when entering this tile. This is of course not what you want, you want the game to open the merchant screen when entering this tile.
To correct this, you need to overwrite the positions in the GAME1_/map01.xml file that have been corrupted. OVERWRITE the position that corresponds to the faulty tile under <actionClassMap> (single dot) with an "a", and OVERWRITE the position that corresponds to the faulty tile under <actionMap> (two dots) with "04". when you are done, the corresponding tiles where the merchant is in the GAME1_/map01.xml map file (of your savegame), will look the same as in the MASTER1_/map01.xml file.
Don't change anything else. Save the GAME1_/map01.xml file.
Back to the command prompt. Type in the following command:
"packgame GAME1_ GAME1".
THIS WILL OVERWRITE THE OLD "GAME1" FILE. Make sure you have made a backup before this step!!

Copy the file back to your savegame folder and you're done. The shop can be entered again when you start up the game.

You can use this method analogously for any other merchant, the clone tank bug, or any other problems with corrupt maps/tiles.
Item [1]
Your current party record will be saved in either GAME1 or GAME2 depending on which location you are currently in. For example, if you are in Quartz, then your current party record will be saved in GAME1. If you are in the Sleeper Base, then your current party record will be saved in GAME2. This info is not needed to correct corrupt maps (tiles) in your savegame, it is general info on how the savegame system works.
Item [2]
For this program to work you must have java (http://www.java.com/en/download/) installed. You must either:
1. add the java executable the system path (not recommended) or
2. edit unpackgame.bat and packgame.bat with the path to the java executable on your system. For example:

Item [3]
The MASTER1 and MASTER2 files in your savegame folder contain the untouched (unplayed) maps, store item lists, and default party (with Snake Vargas, Angela Deth, Thrasher and Hell Razor). When you start a new game these files are copied to create the GAME1 and GAME2 files.
Instead of unpacking these files you could copy the GAME1 and GAME2 files from .\Wasteland\rom\data folder where you installed the game. They are the same in regards to contents as the MASTER1 and MASTER2 files in your savegame directory.